2000 chevy silverado 4.3L transmission clanging while driving

Have a 2000 chevy silverado, 4.3L and a 4L60E. runs fine and shifts fine.no slippage at all, an occasional clunk when put into gear. Start driving and it starts clanging in unisem with driveline rotation. a metal clanging on metal sound and echoing through driveline and a squeaking rotational sound. u joints are tight. thought at first it was my cat going out however it does not rotate. heard best (while in the back of my truck while my wife drove) from between cab and box with head close to back window. Fluid is good. runs like normal but something is definitely wrong…..any suggestions??