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Is yours an automatic transmission or a manual? I had a 1991 K2500LD (6 lug) and I saw a 3+ MPG increase just from swapping from a 4L60 (similar to 700R4) to a NV4500. It is actually posted on here somewhere if you search. I had that truck up to 18MPG with no real serious mods, just the transmission swap and air dam replacement.
Keep the Suburban versus swapping to a truck body or a jeep, having the four real doors is nice. I would probably still be driving my aforementioned K2500 if it had four real doors.
On to off-road ability, you mention you need 35s and a lift. My biggest beef with 35s isn’t the 35” diameter; it is that you are usually only able to find them in fairly wide widths. In addition to additional aerodynamic drag, this also increases the weight of the tire. I would suggest looking at 34” tires, such as the BFG KO2 in 34x10.5R17 or the almost 34 255/85R16 tire size, such as the Cooper ST Maxx. (I have 255/85r16 on my Tacoma) These tires offer lots of ground clearance without excessive width or weight. To make up the difference between the 34’s and the 35’s shave your differentials. Shaving the differentials 1” while on 34s would be like having 36s. Look at military vehicles, their tires are tall and skinny, and some vehicles (HMMWW) are sitting on portal axles to give extra clearance at the differentials.
Also don’t go full on mud terrain on your tires, try to stick to ATs as they handle so much better on the roads and in fact some of their features (sipes) help you offroad more than you might think. Match your air pressures in your tires to the conditions.
Put air deflectors in front of your tires. Make them out of conveyor belt. Also make a conveyor belt air dam. You can drag it offroad and you won’t tear it up, but it will improve your fuel economy and improve your onroad handling. It doesn’t matter how great your truck would have been offroad if it ends up in a ditch on the way there.
I would suggest not putting a lift kit on your truck. All this does is raise your COG without offering you any real ground clearance at the axles. Instead, make skid plates to protect your underbody if you do drag it. These could also have aerodynamic benefits if you make them like a belly pan. Trim or roll your fenders if you have to to fit the tires. Remember lift kits alone don’t keep you from rubbing they just change how often you rub because unless you limit your up travel your tires can still cycle all the way up, and in fact on solid axles with one side in full droop and the other side in full compression taller springs can actually make you rub worse.
If you want lift, put air bags under the truck with valves you can access from the cab, but don’t inflate them when offroad (as this often limits your flexibility and would raise your COG) until you actually encounter a situation you really need the belly clearance (e.g. high centered on your belly or about to go through deep water).
Use close to stock height flexible springs to make sure the truck can flex and keep the tires in contact with the ground. Also get selectable lockers, electric or air operated. This would be a better offroading improvement than anything else listed.
You mentioned an axle swap. If you wanted more strength without going all the way to tons, I would look for a 14 Bolt semi-float 6 lug rear axle from early 90s heavy duty 1500s or light duty 2500s. This would be a definite improvement over a normal 10 or 12 bolt, let you keep your six lug wheels.
Run stock alloy wheels (to reduce weight) with the appropriate offset so you don’t stress your wheel bearings. Respect the hub or lug centric nature of your axle, have a machine shop turn the wheel hub seat if you want to run newer factory alloys with the smaller hub diameter. Have a matching full size spare, used is ok but a dry rotted 30" street tire is not.
Have enough tools to reasonably be able to fix an issue or get yourself unstuck, but don’t bring the kitchen sink. A powerful come along, jack with cribbing, straps, shackes, shovel, etcetera matter a lot more when you are stuck than a lift or bigger tires. Try to watch how much weight you add and where you add it. Wheel easy, don’t just slam and power through obstacles, drive carefully and with purpose. Go slow and enjoy the fact you are outside enjoying a technical challenge of driving through the terrain. Why rush through everything so you can go back home? Engage your brain at all times, as offroading is inherently dangerous, but focus on your surroundings is required. Wheel with another vehicle if at all possible, making sure they have the same goals for the trip as you.
If you haven’t already look at the expedition portal site. They have lots of examples of purpose built vehicles that have the modifications that matter to get them places, however the mall crawlers have started to infiltrate the site so you have to wade through that now to find the meat.
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