If you still have stodk gears there should be a little silver tab attached to one of the diff cover bolts. If it says 2L73 or 3L08 or 3L27 then you have a Traction Lok carrier and need the 4oz bottle of friction modifier. Or you could check the numbers on the driver's side door jam and see if you have a Trac Lok.
an easy way to tell if you have a limited slip (traction-lok) differential is to jack the back of the car up and then put it in nuetral. spin the back tires forward by hand if they both spin the same way you have traction lok if they dont you have an open diff. i hope this helps.
Under the 'AX' there should be a letter or number. For 94 models K, M, R, W, and Z are limited slips and for 95 models E, M, and Z are the limited slip models. This is from a Chilton's manual for 1994-98 Mustangs.