Fluid exchange on diffs, transfer case, and transmission. A quality synthetic 75w90 for the diffs and t case, Toyota WS fluid for the trans. New plugs, a new air filter, and a coolant exchange wouldn't hurt. Grease the zerk fittings on the driveshaft while you're down there.
Replace/upgrade the shocks. Bilstein 5100's or 6112's are popular and a great boom for the buck. But I just installed some Eibach's on one of my rigs and am seriously contemplating putting them on my tundra to replace the 6112/5160 combo...
Add some airbags or a leaf spring kit to compensate for that topper, trailer, and junk in the trunk.
Don't run 85 octane that we get at higher elevations. The truck will run MUCH happier. I ran 85 for the first 100k with a few exceptions. Switched to 87 or higher and it idles better, has better of idle response and torque, but makes the same power and mileage.
A good set of tires if you don't already have them. Pick the poison that suits you best, but I've found a quality set of all terrains to yield excellent results. Michelin LTX if you don't want something aggressive; LTX/AT2, Big O At's, Cooper AT3's have all served me well. On my second set of Cooper AT3's. Went back to stock 275/65R18 after 275/70R18. With your 20" wheels, I think the stock size is 275/55r20 or thereabouts. Might be a 255 or 265 option.
After all that, the 80 MPH interstate speed limit will suck gas. But sometimes I gotta get where I'm going so I take the fuel penalty.
Enjoy the tundra!
