Although all s10's except the ZR2 came with the 7.5 rear, the easiest way to tell them apart is by the 4:00 & 8:00 positions as viewed straight at the cover. The 7.5 has little "fangs" where the 8.5 has larger triangles. The 7.5 had 26 splines before '88?? I think that was the year they changed to 28 splines.