It seems like you might be building the diff incorrectly, or something could be binding because from what i heard, the rb5's diff's are the bomb. I would take the diff out of the tranny and totally rebuild it with the manual in front of you. Try to use carbide diff balls as well instead of the steel ones. And the diff should not be locked when running the car, back it off about a 1/4 of a turn. Make sure your slipper is set as well.
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