with 33s its more about tire and rim width and offset and bumpstops. a xj will flex so much that 33s will hit on any lift if you don't extend the bumpstops. the best stock look is to take the flares off and do a little trimming then put them back on slightly higher. 2 doors have more room in the rear than 4 doors to do this mod. used to be local xjs all ran 33x9.50 bfg mudders but with the newer lifts i see mostly 12.50 now. i'm gonna be happy with 32x11.50 on 5 inches of lift.(currently at 30x9.50 with 3-4 inches depending which corner you measure)
I run 33x12.5 swampers on my XJ with 7.5" lift, plus generous fender trimming and with the flex I get they still rub. WIth a 6" lift you could can run 33's, but just expect to have some rub. Yes you will need to do some fender trimming unless you put in good sized bumpstops.