Sagging headliner and are newer years compatable w/ my '91?

I searched and found a few threads about where to buy material, but if my material is in good condition, is it necessary? Do I just remove the headliner board and reglue the factory fabric or should I buy all new foam and material? Also, I have a '91 and I love the speaker location of my brother's 2000 (on the roof in the rear cargo area). Are those seperate pods that are just bolted to the headliner board or is it one molded piece? Would a 2000 fit in a '91? What other years came w/ that?

Yes any 84-01 Cherokee headliner is interchangeable.

Here's a newer headliner with molded speaker pods in an early XJ. It looks to be a 1989 or older since there are rear seat shoulder belts (just the holes where they would mount).

Are those speaker pods seperate from the headliner board? Just curious if they can be mounted onto my board or do I need the newer one? Also, is the headliner fabric like normal headliner fabric or is it custom molded to fit w/ the speaker pods? My brother just bought his '00 yesterday and I only saw it for a few minutes at night, so I haven't had a chance to have a good look.

The headliners in the 98+ XJs have the speaker pods built into the headliner (like the one you see pictured). Starting in (IIRC) 94 through 97, XJ's had an optional speaker bar that mounted on to the surface of the existing headliner.
If you are simply looking to add overhead speakers to the back of your XJ, the speaker bar is a much easier way to go.

Yeah, I have an 85 and I vote for the speaker bar. Just check around the junk yards, you should find one. They work beautifully.

As for the fabric, you should probably buy replacement fabric. It generally has a foam backing - which, in the case of a sagging headliner, has become detached from the fabric. When you replace the headliner, you have to use a brush to remove all the foam backing....in order to for the new headliner to adhere properly.