I carry all the tools I need to fix ANYTHING on the Jeep (though they may not obviously be the optimal tools - they would get the job done). In addition, I carry spare u-joints, TRE's, and depending on the trail - axle shafts. I also carry the basics such as RTV, JB weld, ductape, bailing wire, several ratcheting straps,e tc. The tool box itself is not as big as you would think - it's about as wide as the tub of the Jeep, and about 12" deep and about the same tall. Carefully packaging what goes where can really shrink down on the size of things. The box I use is a military suprlus rocket box - which you can get for about 20 bucks. They're tough, AIR tight, and with a fresh coat of non-*******/drab-green paint, such as a hammered finish dark grey, look great. Locks can be added easily, and you can bolt them to your floor as well.
One thing I SHOULD carry but really dont is extra fluids - I just hate the mess they make, and they never come in good sturdy containers. 3 quarts of coolant, 4 quarts of engine oil, a big jug of gear oil, 2 quarts of auto tranny fluid for the t-case along with 4 quarts of syncrhomesh for a temporary fix on the AX-5 = shitload of fluids. Add in the suggested backup of 5 gallons of gas and 1.5 gallons of water per person per day and it's just too much.
If you have a steady group you wheel with - you can coordinate who carries what. One guy can carry half of the tools, another the other half, another spare parts, and another some more (by that time you're at a crew of 4 - you really don't want to go any bigger on a hard trail). One of the benefits of wheeling with vehicle similar to you is that parts are interchangable. Remember that with most of the four-wheel-drive suggested 'bring along lists' is that they're HUGE and cover any type of situation in any type of terrain; they almost have you bring the kitchen sink from your home. Over time you can start to trim down a bit once you know what you need - depending on your driving style and the type of wheeling you do. If you keep your vehicle in tip top shape and only go wheeling when things work right you're not gonna be blowing up any engines or cracking your frame in half. If you keep a responsible foot - chances are you won't blow up your tranny and t-case either. The stuff to prepare for is that the things that commonly break; I wouldn't bother rebuilding an engine on the trail, just strap it out (wheeling with a small group is MUCH more important than how many jeeps you can build from scratch out of the back of your trunk for this reason)