Depending on the amount of trimming you want to do, you could fit 40's if you wanted. I fit 31's on my XJ stock, and it was a tight fit, and I know ZJ's are even tighter. But with the right amount of trimming, they should fit no problem.
2006 LJ - 4.0L, NSG370, 31's
2001 XJ - 4.0L, AW4, 3.5" lift, 31's
1997 TJ - 2.5L, AX5, 2.5" lift, 32's - Sold
1983 CJ-5 - Stock - Sold
[URL="http://www.jeepforum.com/forum/f9/tj-tech-boot-camp-must-read-new-tj-owners-forum-members-722109/"]TJ Boot Camp[/URL]