Schumacher sell a complete assembled diff assembly U3338. You guys are spending ages over this issue which sounds very frustrating, why don't you just cut your losses and drop in a fresh diff.
You can then completely strip the diff which is slipping, bag up the components and use those parts as spares when you come to rebuild one again.
Im sorry I can't suggest much for fixing the slippy one, but without it infront of me and myself repairing it I find it impossible to diagnose it.
Id be very willing to give it a complete rebuild for you if we were ever at the same meeting though.