Swapping ground and hot wires on an 1157 bulb would be hard to do as the ground is thru the socket. In order to do that, you'd need to isolate the whole taillight housing from it's attachment to ground. But a bad ground could easily light up both left and right bulbs. I would be double checking ground paths on both housings, including being absolutely sure that all paint is removed between the housing - bracket joint, the bracket - bed joint, and bed - frame.
