All XJs with the towing package from 1987 -1989 came with a Dana 44, all others that year had a Dana 35. The only good thing about the Dana 35 from that year is that it should be a non c-clip axle. If it is a 35 the diff cover will look pretty much round. A 44 will have more angles to it. Here is a good link to help you detemine what rear end you have
http://www.g-body.org/tnt/diffs/diff-housings.php.
X2 to what was said above about the ABS. If you have it tear out the lines and remove the ABS fuse. Ideally I would reccomend replacing the whole break system with a WJ booster and master cylinder (look for writeups on it). I plan to do that eventually but my jeep is not my daily driver so I am still using my crippled ABS system for now. If it did come with ABS I would not be surprised if someone already tore it out.