One thing I always did to my cars is this...
- Get some old nasty bearings or bushing for the diffs as well as old nasty diff balls.
- Assemble the diffs and drivetrain (don't need to mess with anything else
- Fill the diff cases with nasty cheap toothpaste (equate paste from Walmart works well and leaves your diffs minty fresh)
- Attach a motor and let it run at a low voltage 2-3v for about 20 hours
- Tear EVERYTHING down and clean it
I've done this to break in the gears on the last 4 AE Shaft cars I've owned.