If you ever have a car that's doing weird things in the middle of the corner, like spinning out unexpectedly, make sure the crossmember the 4 small gears in your diff sit on is completely seated. It's tempting to spin the gears before you assemble it which can push them out of place and make weird things happen. This message was brought to you by two weeks of lost time.
Also, I can't take credit for discovering this, that belongs to someone else.