To replace the diff. I believe you have to replace the transmission as a whole.
The hardest part about that is finding a 91-92 9000 tranny.
Normally can be found for around 300 bucks and just have to add shipping to that cost.
Replacing the tranny takes about 8-10 hours because you have to drop a portion of the subframe.
Write-up. http://www.quasimotors.com/clutch.htm