Nothing serious so far. I have had the U-joints replaced in the front axle and the front driveshaft.
The rear window didn't work when I got it and it still doesn't.
I am having the gas tank support and frame repaired due to rust, a very common problem in the city because of all the winter salt.
I love my Jeep. This is the third jeep that I've had experience with on a long term basis. This is however the first one that I own, I specifically wanted a Grand Wagoneer for the comfort and style - love it or hate it, it does evoke an emotion when you see it, more people tell me they love it, wood siding and all.
A lot of people complain about their rear windows not working. If the switch up front and the wiring is still good you might check the little locking mechanism in the inside the tailgait. It is wired so the motor won't work if the lock isn't unlocked. The local shops said it was going to cost me an insane amount of money to rewire and fix, but I ended up fixing it myself for next to nothing. (splicing a few wires) I hope this helps out a few people.
I'd like to second the above comment and also say that a common area to find a short in the wiring is very near the hole where the wires come into the tailgate. The wires actually run on the outside of the car in the back -- underneath the body along the frame. The best way to deal with this is to remove the panel that covers the inside of the tailgate. Most problems can be solved from there w/o much effort.
These two previous tips are right on. Be sure to check that switch located, (as you face the rear of vehicle) it will be on the drivers side close to the alignment pin receiver. This switch does not need to be functional and bypassing it may indeed correct your problem. If the switch has gone bad it will result in an open circuit rendering the keyed tailgate switch and dash switch both inoperable. If connecting the two leads together doesn't restore operation of both switches, I advise checking the keyed tailgate switch itself. The stock component is comprised of phenolic plastic (same type of plastic used in distributor caps and other electrical applications as it is non-conductive.) This type of plastic is also fairly brittle and age contributes to this. As well the pole positioning lever located inside the switch body is plastic as well, although not phenolic. Obviously the fatigue of movement and stress on these will lead to breakdown. I know the replacement part is available from the dealer and isn't all that expensive (around $40.00). Well worth it as a wagon is greatly limited without access to the cargo area. In closing I should mention I'm the proud owner of a '91 Grand Wagoneer and she is near and dear to my heart. I'm pleased to see all these testimonials and feel the obvious warmth and compassion we all share for this truly distinguished American icon.