I have the automatic transmission 4 wheel drive quad cab and when i go down a hill driting and start to go up another hill and give it gas there is a clunking noise somewhere in the driveline it is like there is excessive backlash in the rear or something. The truck only has 6000 miles on it

 

i'd check your u-joints first. you could put the truck in park, engine off, and jack up the back end of the truck. try to turn your tires back and forth. might be able to isolate it by doing that or even put the tranny in neutral before you jack it up with the front tires chocked.

and can we get some more info on your truck? it may give others better ideas.

 

i had this on mine recently, along with vibration and a squeak at low speeds. turned out to be the last u-joint in front of the pumpkin.

dealer put an aftermarket , grease able replacement in it's place

Bryan