The mechanic checked ujoints, rear differential, brakes and found nothing. He said he thought it was the transmission downshifting from 2nd to 1st gear when the noise is occuring. He said to take it to the dodge dealer becuase he could not find anything wrong. I really don't want to do that if its just a natural noise but I know for sure the noise did not start happening until two weeks ago and I have owned the truck for 3 years.

 

I'm bringing this back to the top as I still have not figured it out. If I take the overdrive off or put the tow/haul mode on there is no clunking sound.

 

You might try posting in the diesel forum. Is anything different happening with the RPMs during the deceleration when overdrive off compared to overdrive on?