[Big Dave]

I concur with the idea of changing to a 3.42 R&P set.

That will slow down the engine speed by 17% at any given road speed. That will reduce the amount of air your engine has to pump and will reduce engine frictional HP. That approach certainly works for me. I'm getting 24 MPG in a tough winter with heavier truck.

I assume you have a NV5600 gearbox. Should be bulletproof with the 3.9 engine.

After you square away your gears, start thinking aero. Grille block, an air dam if possible (not always possible on a 4x4) and DEFINITELY some sort of bed treatment - either a tonneau (hard or soft)(good for +1.5 MPG over open bed for me) or better yet an aerolid of some description. A crude aerolid garnered me 3.0 MPG over an open bed.

Even as a 4x4, you should be able to get that truck into the low to mid 20s driving the posted limit, even in hilly coastal Oregon.