I would concentrate on things that will help you get back to town.....
-PRINTED FSM in the vehicle at all times.
-Code reader, see the FSM for details on the codes
-Serpentine belt, that fails and your pretty much dead in the water theses days....alternator, power steering, maybe the cooling fan, etc.
-Upper radiator hose and/or a wrap kit
-Some spare fluids, but not a complete change of fluids. I suggest ATF, engine oil, and gear lube...maybe some blinker fluid
-A decent but not extensive tool kit. Its far to easy to go overkill. Look as what spare parts you carry, determine the tools needed for those repairs, throw in a few universal tools for 'what-if' type stuff.
You can do a LOT with a little IF you HAVE to.....
Specific to the JK....
-Have the ability to pull the front axle shafts and remove the front driveline. Ideally having spare front shafts would be good, but for the weight I would upgrade to alloy or CV units if your that worried about it. I am pretty sure the JK uses a captured unit bearing in front that does not require the axle to be bolted in place like the older TJ/XJ unit bearings? If you pop a front axle u-joint it will most likely stretch the ears out so a replacement joint really isn't much of an option on the trail. You should probably pull the outer shaft and joint out to keep from further destroying the shaft and having it be almost impossible to remove later. Then wire/ziptie/tie the inner shaft(s) in place to keep the gear lube in the differential. You can still use 3WD if you have a locker in the front if that will help get you out. Or you can remove both outer shafts. On a TJ you use to have to keep the outer axle stub shaft in place to pre-load the bearings, what a pain in the rear.
-Be able to remove the driveshafts. The is very helpful when dealing with front end locker, axle, or ring and pinion issues.
-The rear axle is pretty durable. Bent flanges are the most common problem but the vehicle will still remain mobile. Popping the driveshaft at the pinion yoke is another one I have seen more than once. Basically tag a rock just right and that joint can see a lot of stress. Being able to remove the rear driveshaft is handy for limping off the train in FWD. The JK uses a CV joint in the driveshafts for the most part, that might be a field replaceable part but I haven't seen anyone swap one out on the trail.
If I was going to be REALLY prepared....
-Fuel pump. It never goes out right up until it does. I have seen a few vehicles take hard impacts in the fuel tank that killed the fuel pump deader than dead. Our Audi fuel pump decided it didn't want to work anymore the other day...just stopped dead. Left us stranded in a nice little cell-phoneless canyon on the way home. Only lasted 190K that dang thing!
In general most modern vehicles when rather new and maintained wall are VERY dependable. I would be more worried about damage from an extreme situation off road. Axles, brake lines, driveshaft, etc.
