O.K., different diff I was thinking about. On diffs with four spider gears, there is a pin, that "keys" the large bevel gears that go to the outdrives. yours is the MO5 type.

So, if the bevel gear on the driveshaft is O.K., and not spinning on the shaft, Like kstmRYD said, check your bearings that have anything to do with the diff, itself, and the wheel bearings on the knuckles. If it isa bearing,at least the work was notin vain, as you have pretty much done the proper maintenance on everythingelse!