You guys do know that the proper way to do the wheel spin test is to hold the driveshaft still, right? If you have an open diff with tight tolerances and there's very little friction on the driveshaft or the output shafts, you can turn one output shaft and the other one will turn in the same way.

The best way is to hold the driveshaft locked (trans in gear or in Park) and try to turn one wheel. If it's freakin' hard to turn, you either have an LSD or a brake problem. :-) If it turns easily, it's either an open diff or it's an LSD with burned-out clutches.

I never checked my '87 GXL. The only FC's that I thought to check were an '86 base model (open diff) and another '87 GXL (open diff). I wonder if there's a similar situation with FCs as was with the FBs, where people would complain about noisy LSD operation and the dealer fix was to swap in an open diff?

 

Dak is correct about the 89-90 GTUs being the only ones available with a 4.30 gear in the rear.

A wore out clutch type differential will act just like an open differential. The clutches disk are slipping(just like wore out clutch in a transmission) and the differential appears to be an open diff. The fix is to rebuild the rear with new clutch disk. Thats cheaper than a new gear set but since your in there might as well get the 4.30 gear and install it.