A good, fixed blade knife should be one of the first things you put in your truck. Don't get some whimpy one; this is your chance to get something Rambo would carry! A knife can be your most important and utilized tool when in the sticks, and you don't want some cheesy as-seen-on-TV special that will snap the first time you put a little side load on it. We state fixed blade because they're just more heavy-duty and safer than a folding knife. You should probably have a folding knife on you too, but you need a bigger, fixed blade to handle big tasks. This Benchmade Snody-designed fixed-blade knife has a blade just over 5 inches, and features partial serration. Having a straight and serrated blade gives you the best of both worlds when it comes to cutting something.
You should have a bevy of tools at your disposal. Extra parts are smart too. If you don't like carrying an entire tool set, then carry sockets and wrenches for the bolts that are on your suspension and drivetrain, plus some pliers, screwdrivers, and adjustable wrenches for anything else you forget about. If you're not carrying entire driveshafts and complete inner and outer axles for the front, then at least carry U-joints, lube, U-bolts or straps, extra belts for your engine, extra hoses in case something blows, and multiple fuses. Wire would be a good idea too, but you can always pirate off your own taillight wiring if you need some wire badly enough.
Having a good flashlight is important too. We know a lot of you have been carrying around the same model flashlight for the last 20 years; it doubles as a good beat stick, and it's hard to change old habits. But we're now into the 21st century, and there are some great LED flashlights that will last for over 100 hours on a few AA batteries. They're lightweight and easy to carry, and will not take up much room, though you'll have to carry a separate beat stick if you want to whack anything. Bring a ton of spare batteries as well. Matches and multiple lighters should be standard in every 4x4. Keep 'em in a container or baggy that won't let them get wet either. At the very least, you need these to light the M-80 and other firecracker "signals" that so many guys seem to have in their trucks these days...
We won't even bother mentioning a winch because you know you should have one, so let's skip right to the recovery kit. Most recovery kits like this one from Warn should have a snatch block, a tree strap, some gloves, a shackle, and some chain. If you get creative, there's almost no situation you couldn't get yourself and your truck out of with these things (as long as your winch is still running).