Did you use the small greyish plastic looking shim that goes between the rubber o ring and the inside of the diff? I had the same issue. When I rebuilt mines the first time with the traxxas metal drive shafts i used a shim on each side. When I rebuilt my diff for the second time I used a shim on either side and noticed it would not turn freely. I removed a shim and it worked fine spinning free. Dont leave the screws loose just remove a shim if you have too.