Swapped a 8 3/4 into my Doba. Bought a 3:91 sure grip in a 657 case. Clutch style sure grip unit. Got it on the ground and it won't move. I looked under the car and the driveshaft spins. I jacked up the rear end enough to get the tires off theground and put it drive and it works fine. What's goin on? It was supposed to be rebuilt but I have no proof and the seller claims it worked when pulled. I'm upset and annoyed. Please help.