i definitely say stick with the 33's for the lift you have, and get the bfg km2's.
if you went up to 35's would definitely need to regear the axles. with 33's can live with the stock gears for a while. not to mention the need of more lift for 35's and other headaches that come with them.
your xj has the d35 in the rear, definitely dont want to run 35's on that axle.
to get to 35-37 range on the xj will need: gear change, more lift, change rear axle, and probably rear d-shaft.
I have seen xj's on 33's do well in a lot of places, had one in a group of us at the hammers, with driver skill(bravery haha) he followed the 35+ wrangler crowd with no problems.
