ORIGINAL: HPI_Savage_RC
Sounds like you damaged the spider gears inside the diff assembly. The only way to verify the problem is to disassemble it and take a look.
I took it apart and the gear that is attached to the drive cup (I believe its called the pinion) was broken in half. The rest of the gears appear to be fine. However, the pin that holds that gear to the pinion was missing and is nowhere to be found. Weird.