Before a long trip, I usually do a complete once over. Just finished the latest of several 4000 mile + trips with only separated tire failures to cause trouble. In fact, I've not had a significant mechanical failure in many years of moving/traveling all over the country from coast to coast (knowing full well that saying this guarantees a disaster on my next trip).
Check all fluids. Particularly make sure you have fresh coolant with summer temps to prevent boil over. If oil is getting "up there" near a change anyway, do it early and change filter. PS, brakes, auto trans, axles, etc. should be visually inspected for level and condition of fluids.
New air filter if condition is at all suspect. It's cheap and can really hurt performance multiplied by miles so that the filter is effectively free with today's gas prices. Also check fuel filter.
Grease and inspect all ball joints and suspension/steering components. This failing can ruin your trip and your life. If you don't know how to check the ball joints, have it done by a shop. When I was 17 I had a 72 L82 I built from a basket case. Just finished a 50 mile run at 90-100 mph (illegal of course) and the left front tie rod end *fell off* as I turned into a parking lot at the end. /forums/images/graemlins/eek.gif I knew it was loose, but had no idea it was that close, and being a broke (and stupid) teenager, I "let it wait"…
Inspect brake components for wear, this includes flex lines. Also inspect hub bearings for play, repack if it's been a while and condition unknown. Any suspect brake component should be replaced before a long trip. This is not something you want to have hit you on the interstate in the middle of the night.
Check all hoses and belts for signs of trouble. Also check the radiator cap. If any signs of trouble, replace! I can't tell you how many times when I was younger that procrastinating on belts and/or hoses left me stranded. Look for signs of leakage around the radiator and/or water pump weep hole (stain indicates impending failure).
Check the fan clutch and make sure it's not getting weak. Also look for play in the bearings which can lead to rapid failure.
General once over with a hand full of wrenches (and visual inspection) on all critical mating surfaces which are reasonably available. Just hit all the obvious stuff you see underneath and in the engine compartment. I almost always find something loose on my trail rig.
Set tire pressure (perhaps a bit higher than usual, check Grim's post for finding ideal road pressure based on temps), and maybe rotate as well. Balance and align is also a good idea if it's been off-road much.
Visual inspection of all suspension components. For instance, the 52" springs are notorious for breaking support leafs and it can be hard to spot. Front mains have also been known to break. Also look for excessive wear on front shackle bushings which are often neglected and if anything looks suspect, check the front shackle bolts which often fatigue/wear and break.
Carry basic tools along with water/coolant to refill the radiator, brake fluid, trans fluid, etc. I even carry a spare fuel filter in my diesel. Also carry some "form a gasket" and other general purpose "band-aids" like duct tape, JB Weld, Mechanics wire (aka bailing wire) and the like.
Check electrical system (alt charging, clean terminals, battery condition, etc.) and carry spare fuses and a roll of wire with strip/crimp tools and supplies.
That's all that comes to mind…
