For the engine............reroute the pcv, egr blocker plate with Finger stick, free flowing exhaust, good tune, lift pump, and some gauges (EGT and boost)
For the trans...........if towing alot, Mike L cooler and a deeper pan.
Transfer case............Pump rub kit
Front end.....heavy dity tie rods and inspect all steering components
Rear axle..........aftermarket diff cover for added oil capacity and cooling. Inspect gov loc while in there.
Check the lower radiator hose, there is a wiring loom or something that rubs a hole in it sometimes. Change power steering fluid (makes steering and brakes work better) use a Tech2 scanner to do an ABS bleed (you won't believe how much this can improve brake feel)
I'm sure I'm forgetting some things, maybe someone else will chime in.