After my truck sits for a while, it makes a clunk noise when backing up.
It only does it once when it is cold, and sometimes does it in forward after sitting for a day.
The noise is coming from the rear axle area and sounds like a heavy metallic clunk. Kind of like if you were to tap a hammer on an axle or a real heavy metal object.
Could this be a U-joint?
It doesn't sound like the clunk a loose shock would make.
The truck only has 19k on it.

 

My old blazer did this too..I always chalked it up to moisture getting into something and freezing, but I'm probably wrong. It will be interesting if anyone comes up with something