Cherokee started using the 29 spline in late 1996, so yours should be a 29 spline unless someone changed the rear diff and installed a 27 spline axle.

Get an Aussie locker, it is a lunch box type locker and you can install it yourself in half a day or less.

Don't waste money on a LSD! They are worthless off the highway, and you would need to setup the diff for it.

 

I know LSD are not popular on this site, but I replaced a Powertrax in my 98 with an Auburn LSD. I have been very happy with it. It is my daily driver with some off road, and mud in the Pine Barrens. The powertrax never worked right. Everyone here said I broke it when I put it in. Funny it came out in one piece. Now the box sits in the garage collecting dust. For about the same price I paid for it, now I have a rear that works for me. If I were going to do more hardcore wheeling I would buy a real locker.