^x2. sounds like a brake is sticking when in reverse rotation and is energized or locked up which is why I presume your wheels look the wa they do in the pic. So, when the brake power is disconnected all is fine? If so, its narrowed down to a brake issue. Keep us updated.
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)