I'm sure most people will answer with something like 'it depends'

- which is true. Are you a 2 min walk to a nice road or 3 days?
First, do regular maintenance so things on a trail/trip are a real surprise (vs. 'oooohhhh yeah, my rad hose was kinda shot...I should have done something about that')
Assuming you are talking about parts (and not tools, first aid kit, fire extingisher, snatch straps, and emergency stuff like water, food, matches, blankets, etc) and not major parts (eg spare starter, bearings) then you could start with things like:
- Factory Service Manual - important 'part' of fixing things, or at least good reading while you wait for someone to show. Many 100 pages, so you can eat it in a pinch

- Spare gas
- Spare belts
- Spare fuses (automotive fuses are cheap - buy a variety) but make sure you have fuses for the vital parts that keep your truck running (vs the radio and sunroof)
- Spare oil (to full oil change and filter)
- Extra coolant
- Extra tranny and diff oil
- Brake fluid
- Rad hose
- Some spare fuel line
- Spare lights (headlight, tail, etc)
- WD 40
- JB Weld
- Hose clamps
- Zip ties - assorted lengths
- Good duct tape
- Good electrical tape
The list is endless. The longer the trip and the more remote it is, the more stuff to take. Search the net - lots of suggestions out there:
http://www.geocities.com/george_tlc/#checklist
http://www.exploroz.com/infopages/parts_and_spares.asp
http://media-north.com/4wd/mtech.htm
Cheers, Hugh