You didn't mention if this was a 4cyl, V6 or V8. I don't remember if the fox body GTs came with something other than the 302 V8 or not. If it's the V8, it will already have a limited slip differential and it's just worn out. If it's anything but a V8, it will have the anemic Ford 7.5" rear axle. If that's the case, you will want to get a different axle assembly all together because the 7.5" is a very weak axle.