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Going by this page over at 4crawler.com, I will see how close to vertical I can git.
Toyota 4Runner Suspension Upgrade

Shock Mounting Angle examined:
One benefit of angling the shocks inward is that this allows for increased vertical travel for a given length of shock. If a shock is mounted vertically, it needs 1" of travel for each inch of wheel travel. But angled inwards, at say 45°, the shock travel is reduced by 30-40% over a vertical setup. With the 14" travel shocks I selected, I am able to achieve over 19" of vertical wheel travel (assuming my springs would allow that). A drawback to this arrangement is that the effectiveness of the shocks is reduced by the same amount the travel is increased. Since I chose an adjustable shock, I can compensate for this by "cranking up" the rear shocks for added stiffness if needed, more on this below...

Rear shocks, muffler and dual batteries

When mounting a shock absorber at an angle other than vertical, several things occur simultaneously that affect the shock's damping properties. The first effect is due to the static angle. When a shock is vertical, you get 100% of the shock's damping action acting to control the suspenion's up-down movement. When you start tipping the shock over at an angle, the vertical damping effectiveness falls off in relation to the sine of the angle the shock is angled at (assuming 90 degrees is a vertical orientation). Looking at some common angles and their sine, we see:

sin(90) = 1.00
sin(75) = 0.97
sin(60) = 0.87
sin(45) = 0.71
So, what the above numbers tell you is that if you angle the shocks in at say 45 degrees, they are only 71% as effective in damping vertical motion as they were if mounted vertically. That is, if the shock generated 100 lbs. of damping force at a given velocity, only 71 of those pounds would be directed upward to resist the suspension motion. An equal amount would be directed horizontally (i.e. pushing against the frame sideways) and that force would not do anything productive. This is a static factor affecting the shock absorber effectiveness. Note that this would also apply to say a coil spring mounted at an angle as well. The more it is angled over, the less force it can apply vertically.

However, that is not the whole story. When you mount the shock at an angle, it changes length at a slower speed, for a given vertical motion, than if a shock is mounted vertically. If you look at the extremes, at 90 degrees there is a 1:1 relationship of the suspension moving up and down and the shock compressing/extending. If you were to lay the shock on it's side (i.e. 0 degrees), the shock would not move in and out at all as the suspension moved up and down. Once again, the relationship of the shock's velocity relative to the suspension's up-down velocity is related to the same sine function as above. When the shock is vertical (90 degrees), the ratio is 1.00, when the shock is at 45 degrees, the velocity is approx. 71% that of the suspenion's vertical speed.

sin(90) = 1.00
sin(75) = 0.97
sin(60) = 0.87
sin(45) = 0.71
Note:
You can see this visually on the following web page:
-> http://www.saltire.com/applets/triangles/tri2sia.htm
If you set the sides of the triangle to 10.0 units each and the angle BAC to 90 degrees, this will represent the length of the shock (on the diagonal) at 45 degrees (it'll be upside down as shown). Then "nudge" the AB distance by setting it to 10.1 units (a 0.10 unit change in length) and notice the "shock" length changes from 14.14 to 14.21 units, or a change of 0.07 units or 0.7 times the rate of change of the vertical dimension.
So what does velocity have to do with anything? Well, a shock absorber, assuming a common hydraulic design as is commonly used in vehicles today, is a device that generates a damping force that is proportional to it's speed or velocity. Ignoring friction effects (like the seals inside the shock) and spring effects (such as from internal gas pressure) and variable damping rates (as from progressive shocks), if you compress a shock twice as fast, it generates twice the damping force. Likewise, compress (or extend) it half as fast and it generates half the force. OK, so where does this take us?

With the shock at an angle, it moves slower, relative to the suspension's up-down motion, and the more it is angled over, the slower it moves. Thus, if the shock compresses or extends slower, it is creating less damping force for a given suspension speed. So this is a dynamic factor affecting shock absorber performance.

Going back to the first part of this discussion, you need to combine the static and dynamic effects of the shock's mounting angle to estimate the overall effectivenss. Since both effects are independent, their combined effect can be calculated by multiplying the factors, which in effect squares the values in the above table

sin(90)^2 = 1.00
sin(75)^2 = 0.93
sin(60)^2 = 0.75
sin(45)^2 = 0.50
So we can see from the above table, that the shock effectiveness falls off much faster than could be accounted for strictly by the static effects of the mounting angle. And if you look at going from say 45 degrees (0.50) to 60 degrees (0.75) the shocks would feel 50% stiffer at 60 degrees vs. at 45 degrees (0.75/0.50 = 1.50). And going from 90 degrees to 45 degrees, the shocks would feel 1/2 as stiff. The above is a long-winded way of saying that as the angle of the shock decreases, it pushes less hard less efficiently in the vertical direction.

So, what is the take away from all this? Mount the shocks as close to vertical as possible for maximum damping effectiveness. However, you need to temper that requirement with physically fitting a given shock into a given space. Afterall, the axle is only so wide and that limits how far apart the shocks can be placed. You only have a given amount of vertical space between the axle and the frame (or upper shock mount) so that affects how much vertical separation you have between mounting points. If the vehicle is being set up for off-road use, you probably don't want to have the shocks hanging down below the axle (rock anchors). And, unless you are planning to cut holes in the bed and run the shocks up through the floor (in which case you have unlimited room and can do whatever you want, shock length-wise), there is not a lot to do to increase this vertical separation.

So why not just use a short enough shock to fit in the space given? Shock travel is usually the answer. You want to have a shock that can extend far enough to not limit the suspenion's travel off-road. Afterall, you probably just laid out a bunch of money and/or time putting on the flexiest springs you could afford, so why have the shocks limit what those springs can do? OK, so put in a longer shock. Well, if the shock is too long, it can limit the compression of the suspension (not to mention damage the shock), so that is no good either. To get more travel out of a shock, it needs to have a longer rod. In order to allow the longer rod to fit into the shock body, the body itself needs to be longer as well. This is a dirty little secret of shocks, that in order to get 1" more travel, the shock's compressed length also increases by 1", and this is on top of approx. 5" of non-useable compressed length in a typical shock. So a 5" travel shock is about 10" long (compressed) and a 10" travel shock is about 15" long (compressed) and so on.

If you only have so many inches of vertical space to mount the shock and you need a certain amount of vertical shock travel to accomodate the suspension articulation, you have to work the numbers to find out what works. If you can find a shock that is short enough to fit the space and that also has the required travel, you are golden. Put them in vertically and call it good. However, if you have a relatively low lift that flexes quite well, you may find that in order to get the shock travel you need, the shock's compressed length is too long to fit. So this is when you resort to angling the top of the shock away from vertical. How far to go? Ideally, find the angle that is just enough to allow the shock body to fit with the suspension fully compressed (this way the suspension bottoms out before the shock does). Then make sure the fully extended shock is long enough to allow for full suspension doorp/articulation. Luckily this part usually "just works out", because as the shock is angled over, the effective extension length is increased since the diagonal distance increases less than the vertical length does. So, what you lose in damping, you make up for in length.