These things are geriatric at this point. The one issue I worry about is the fact that all the plastic and rubber parts are brittle and fragile. Can't really do anything about that.
The two biggest weaknesses of these vehicles are the transmission solenoids and the timing chain guides. There are two timing chains and it's a big job to fix them, but if they go it ruins the motor.
When I got my 05 4.0, the first thing I did was change every drop of fluid in the thing. The trans was bad from the get-go so a trans shop did that.
I did the oil, differentials, transfer case, brake fluid, power steering fluid (both brake and PS fluid were solid black). The only thing I didn't change out was the coolant because A) it looked fairly fresh and B) a nagging suspicion that someone used stop leak in it and it wasn't leaking so I didn't want to rock that boat. Got a new battery and tires so I didn't have to worry about either. I also changed the oil filter, air filter and fuel filter. The fuel filter was full of junk that looked like used motor oil.
Brakes and rotors were swapped out. Good quality brake pads are a must. Bleeding out the bad brake fluid and new pads made it stop MUCH better than it had been. Check the brake lines. You might also want to do the plugs and wires. I've had trouble with the front sway bar links breaking so check those. Not sure why this has been a problem but the previous owner may have installed them improperly and since been corrected.
In vehicles of these age, basic maintenance is key. A lot of this stuff may never have been done and if not, it's all way overdue. Aside from this stuff, You might get a little extra grunt by putting a performance air intake and filter on. Beyond that you're getting into custom mods. The 4.0 doesn't have the power the v8 does, but I think it's got plenty as is. Mine runs good and I don't want to mess that up by overstressing a nearly 20 year old truck.
