Weird Clunk from middle of truck or driver rear

[75z28]

I bought my truck, a 2002 silverado extended cab z71 , in October with 56,000 miles on it. It has developed a clunking sound from the middle of the truck to the rear of the trunk some where under the body. At first I thought it was the crossmember or transmission mount but not a thing...  It only happens on rough roads and doesn't seem to happen at speed.  I feel it on the floorboard(at least I think I do).

Any ideas?