In my case I'm not sure about the ATF fluid. It sometimes happens even after driving for a few hours which would lead me to believe theat the ATF fluid is plenty thinned out by then. I've had the same clunk in the rear now for the last 250k. It doesn't seem to get any worse though. PS I don't reccoment you go to the dealership, unless you have money to burn!!..

__________________
85 300TDT 456,000 orig. miles