I have an 06 2500hd 6.0 4wd.. I will be driving for a while on the interstate or anywhere and ill let off the gas and when its kinda going downhill when i let of the gas and it starts gearing down ( auto ) There will be a loud!! grinding noise like something is winding up underneath sounds like its from the tranny or transfer case? its not heat shield hitting it. it sounds like torque converter? only has 70,000 of highway miles and pulling a little bit?

 

probably irrelevant, but all fluids are supposed to be changed at 60K miles. both differentials and transfer case. you are not driving in 4wd auto or 4 hi, r u?

 

Nope. It doesn't have auto 4wd.. I'll get those fluids changed sometime. But I'm not sure it's just weird

 

My Chev did that to me exactly as you described. It was the pinion in the rear end. Had to rebuild it...
I'd start there if I were you...