to find out what rear end u have u need to open ur Driver side door or passenger and look for the company information and look under axle type it should be a X or something like that I believe the stock 2.3L's came with 3.23 gears thats all I can help u with once u get the code being X or What Have You then go on the net look for Ford Differentials used of 87 mustang then it should bring up a site that will show u the codes for what the
tranny is EXAMPLE: X=3.23 y+3.73 ETC. hope that helps u bud L8r