This question does not really have an absolute answer since there are many factors to consider. A couple of major considerations are how the vehicle will be used and the real size of the tired compared to the size printed on the sidewall. In general, tire sizes are actually smaller than the size given on the sidewall of the tire, with very few exceptions.
30x9.5 tires would be the practical maximum size for a stock ZJ.
31x10.5 can work fairly well with a 2 inch lift, with minimal rubbing.
32x11.5 can fit with 3.5 inches of lift, as long as the front bumper area is taken care of, and you check the rear for clearance. I personally have 32x11.5 BFG MTs on my Jeep which has a 3.5 inch TeraFlex lift. No problems at all with swaybars on yet. I suspect there will be some minor rubs after disconnect at extreme travel in the front, while turning, but I have not run into a problem yet. From looking at it, I would say a slightly longer bumpstop can take care of any problems in this area. I have a front ARB bumper, so there is not any issues there, but there will be rubbing if a stock plastic bumper cover is left on. A trim job is certainly needed if that is the case. I have maybe a half inch left in the rear on level ground.
33x12.5 tires can be fit with a 4.5 inch lift. When using a tire of this size, the rear begins to become a slight problem. A few have taken to trimming the area right behind the rear door, since from the factory, that is made as just 'dead space'. Another option is to move the axle back with adjustable control arms, but depending on the move, a new rear driveshaft may be required, or if going too far back, then rubbing may become a problem at full compression in the rear (the axle moves back as it compresses because of the way the control arms work).
I will try and get some good pictures of each of these tire fitments up here soon to show where problems may be found.
If anyone has information or pictures, please contact josh@jeepzj.com
I will be posting a nice table here that shows what sizes are best with what lift soon.
RELATED FAQS
Jeep is a registered trademark, and is not associated with JeepZJ.com