rear end clunk when cold

[psaboic]

Just curious....what would be the most likely cause of a rear end clunk when upshifting from 3rd to 4th? Only does it when it is cold. When all the fluid warms up not only does the transmission shift smoother, but the rear end clunk is gone as well. I also noticed if you rev the motor in park when it is cold, you will get the similar clunk when you quickly lift off the throttle.

This is on my brother's 83 300D by the way, not mine. Checked the flex discs and the are fine. I'm thinking mounts or exhaust system moving, or???