Originally Posted by brent701

This was talked about pretty well a few pages back

Current SC10 diff #'s
Gear - 9827 (RS manual)
Ball - 9853 (RS upgrade parts list)

Current B4.1 Diff #'s
Gear 9827 (FT upgrade parts list)
Ball 7678 (FT manual)

As long as you have the newest updated tranny case. any up there will work.
I know for a fact #9827 will fit in to a B4.1 FT tranny case without doing anything to it. I run that in my B4.1 FT
I know the 7678 will fit in a RS case. I did it for a little while.