Originally Posted by EricJ

I've seen this on another forum and it's caused by people trying to put an old T4/B4 ball diff into the new SC10/T4.1/B4.1 diff case, which doesn't work because the case and outdrives have changed (as Asharus explained). There are two solutions:
1) Buy new outdrives and bearings. This is a fairly expensive route.
2) Buy the T4/B4 ball diff case and a T4/B4 rear chassis plate. This is really cheap (5-6 bucks), but could leave you or a future owner very confused.

I hope this helps.