[elhigh]

You could look at the movement of your tires as they rise into the wheel wells. If you can close any of that gap without interfering with the movement of the tires, it's worth doing. Otherwise those wheelwells are just big pockets full of turbulence, making noise and eating energy that could otherwise keep you moving down the road. In your flexed photos, it didn't look like there would be too much interference if you brought the lips of the rear wheel wells down a couple of inches.

The rear spoiler above the liftgate could help, but experiment with the angles. So long as airflow stays attached it's helping.

An option for aero would be a hitch mounted tailgater with an aerobox built onto it. Build it right so it's collapsible when you get to the trails, move it inside the truck for wheeling.

An air dam would probably yield quick, cheap results, but since you want to keep this thing good for the trails you'll need to do some fabbing to make it easy to remove and stow inside the vehicle.

Like others said, instrumentation. You can't know what works if you don't know how it's working. Having instant feedback also makes it a lot easier to adjust your own driving style. A Scanguage 2 should plug right in to your OBDII port and allow you real-time info on your fuel burn rate, engine temperature (important to know if you're experimenting with grille blocks), etc.