I have a 10 bolt 3 series rear end out of a 92 Z28 with 3.23's. It whines bad and leaks out the front seal. I can fix the leak, but my friend has a 3 series carrier out of a '90 IROC with fresh 3.23's. How hard would it be to just swap the carriers? Rear is off of car so easier to work on.

 

pretty easy, pull the carriers, swap the ring gear, install carrier, check back lash and contact pattern and you're pretty much set and ready to go.