Clank noise when put into reverse or when releasing throtel at high rpms in any gear

Hello all, I daily my 73 scamp with a 318 with a 3 speed automatic and recently there has been a fairly loud metal clank noise when I put the car into reverse or abruptly let of the throttle while moving. I'm thinking maybe it could be a bad yolk, broken u joint, loose transmission mounts? It's not the motor mounts. I'm just wondering if anyone else has an idea I might of missed. (I am aware that I could be something internally wrong in the tranny but I don't wana acknowledge that..yet O:)) it still shifts smoothly when shifting but only when accelerating, down shifts fine.. SOMETIMES.