Did you take the diff apart to check the inside where the balls are? It could be a couple things. Your clutch could be bad, slipper pads are bad or something with the slipper clutch, or the plastic gears in the tranny need replaces. I dont know which one it is because i am not there with your car. I would need to see your car in person to know. I would probably replace all the plastic gears in the tranny including the diff gear not the balls though but first take a look at the slipper clutch.