We just went through this on the XJList, you can probably go with something up around the 30-31" range, depending on the width... ie - dont be putting 12" wide tire on a 6-7" rim, you might be able to get it to mount, but it won't be pretty or work well! With a stock rim you should be able to go to the 30-31" range with a 9.5" width at max. If you go much taller than that, which you can, but then it's going to be rubbing and if you do any flexing then there will need to be some trimming in your future too. Anything much taller than 31" then you're starting to look at lifts on a XJ, the general rule of thumb that I've been told is, if you're going to be going to 33's+ at 12" wide then you're looking at 6+ inches of lift, to get any decent flex.... now with that being said, there are alot of combinations out there that you can probably get away with! ..but you gotta look at the over all picture... how do you drive it, what is it mostly used for, where do you drive it, etc.... Also, if you go too big, then you're looking at gears to be able to push those big meats! ...the 4.0 will push big tires with stock gearing, but the gas mileage will kill you! 
...just another data point!
Later,
Chad
88XJ
...just another data point!Later,
Chad
88XJ
