I have a 77 f150 with a 351m and 3 speed automatic. When I accelerate then let my foot off the gas about 1 second later I feel a clunk, feels like it's coming from the rear end. It feels like someone just rear ended me. It only happens when I'm going less than 20 mph. I have heard multiple opinions like it's just the gas moving around in the tank, it's the rear breaks auto adjusting, it's the engine or transmission mounts...What do you guys think, had anyone else had this issue?

 

you have a u-joint thats about completely shot, get it fixed sooner rather than later

 

X2 ^^^ Most likely cause.