You can get a build sheet from the manufacturer that will tell you whether or not your vehicle came with a limited slip rear from the factory. Of course, that doesn't mean that it's working or even that it's still installed - you have to check that yourself. Friction modifier is cheap, and adding some, or spending a couple of extra bucks on fluid that includes it already won't have any effect on your standard open differential, if that's what you have.
1992 Cherokee Laredo - all stock, DD. Needs some minor rust repair, then on to the mods!
1996 Cherokee Classic - 3.5" lift / 31.0x10.5 Mastercraft Courser CTs / Warn XD9000i - Stripped for parts / junked :(
2004 Grand Cherokee Laredo - 4.7, bone stock - Wife's DD
NJ F-Body Owners Association