I just bought a diff and did not realize it was not from an fb. It looks the same except when you go to bolt it up to the driveshaft it doesn’t fit, it has a pin and it’s smaller. Is there anything I can do?

 

So that makes it an SA axle then. You will also have a prob with the thread pitch on the brake line going to your FB, as the SA pitch is different. Ironically, no problem getting an FB axle/brake adapting brake line, but never heard of one going the other way?
Too late to return it?

Stu A
80GS (with FB rear end)
AZ

 

The other "issue" is that SA rear drums are no longer available.