Inspect your Shock bushings, worn bushings will cause your shocks to make similar noise you have indicated to be your leaf springs. If it is your leaf springs, go to a suspension shop in your local area and have them install leaf spring isolators. Or you can get them at most Auto parts store and install them your self.
Here is a link to what i am describing and it's # 4 and #10. Your being a 2500 Sierra, you most likely have a 4 plus 1 leaf system and your going to need a few more isolators.