well ok lets just start out saying that i acctually do own a GC limited with the I-6 and full time 4wd. as far as the full time 4wd system goes i wouldn't recommend it. this doesn't mean that i've really had any problem with it, i just don't believe it is nessasary and not worth the loss of power. the comment about being able to spin the tires, i have certainly never been able to do that. takes a lot more effort to spin 4 wheels than 2. i would recommend the V8 at least if you're going to go with full time 4wd. i would like the selec-trak system better which has a 2wd, full time 4w, part time 4w, and 4w lo. large SELECtion.
i assume you know the difference between a full time and part time 4wd system. noting this difference if the car has part time 4wd i would take it to a mechanic to check the U-joints before buying. but like Bob said it is an easy fix. also really note the break rotors!!! although i have never had any problems with mine my girlfriend's has had this problem repeatedly. in a bad case the whole car will shudder when decelerating, trust me it's uncomfortable. but to help you out ZJ's really didn't change a whole lot through their production. don't take too much note of what year it is and look at what really interests you.
also i have to commend the infinity sound system and if you like listening to music when you ride i would really look for it. i have it and it's wonderful. good luck on your new jeep!