My XJ has a RE 4.5" lift and 31" BFG ATs. When I turn sharply either way, the tires hit the fenders. From reading around a bit here, it seems that 31's are pretty small, especially with a 4.5" lift. I understand that this can be cured by adjusting the steering stops, but should I have to, with that much lift? Also, how do I stop the tires from hitting the top of the wheel well? The lift kit came with 2" bump stops. Is that just not enough? Or am I missing something simple here? Forgive my dumb questions if I am...

I don't know too much about the XJ, but sounds like either you forgot to install the lift kit or something is actually wrong.
stand up by your front wheels, one at a time, with someone in the pilot seat turning the steering wheel to each extreme. Find where the tires are actually making contact. Are they making contact because they are traveling too far or hitting something that isn't in the right place? Bump stops.... You would have to land on your front end with quite a bit of force to actually hit the bump stops on your axles to the frame, even if you put extenders on there....
If you have a digital camera, snap some pictures of these contact points and put them somewhere on the internet and let us know where to find them and we will try to help you figure this one out....

XJ wheel wells are just small. Every XJ I know has problems with that. Also did you go with less offset rims? The wider tires need more clearance on the side that less offset rims offer. Adjusting the bump stops is a temp fix while waiting to get new rims. Even wranglers with stock rims rub with 31x10.5's.

I've got the tires on rockcrawler rims. I don't remember the exact size and offset (they're 15x8s with around 4 or 5 inch offset I think) but it's lots more than the stock rims...

I'll go dig up a camera and get some pictures of the contact points.

Thanks for helping!