It's pretty easy to tell....Dana 35 and 44 along with the Chrysler Corporation 8.25 are the axles in the XJ. It's pretty easy to tell them apart. The Dana 35 and the Chryco 8.25 have oval diff covers, the Dana 44 is an oblong shape. You can tell the difference between the 8.25 and the D35 in that the D35 has a round bottom and the 8.25 has a flat bottom.
The D44 only came in a select few models (some metric version, tow packages, etc.), so chances are you likely have the D35. I only did a disc brake upgrade on a Currie 9" axle, and I've never seen one for the D35 myself, but that certainly doesn't mean that they don't exist.
If you're not going with tires bigger than 33" I really wouldn't sweat beefing up the D35. Disc brakes are nice, but at the price, I'm not sure I'd even consider it worth it to be honest. You could add a limited slip for traction, but again, its a decent amount of money to throw into a street Jeep when you don't really need it.
EDIT: BTW, with a nice e-mail and your VIN, Jeep will happily e-mail you back a build sheet of your vehicle describing exactly what your rig came from the factory with leaving you with no question at all. I had mine inside of 48 hours.