If one or two wheels locked that forceably, the tire(s) would probably show some skidding on the pavement. Next time this happens (and it probably will) look at each wheel behind the direction of motion and determine which tire(s) locked. That would narrow down the component(s) involved. I am more leaning toward something metalic getting into the final drive gear train.
BTW were you able to keep moving in reverse after the event or did you have to jocky forward before coninueing in reverse?