Sparky, The shim in question was originally part of the side mount fender/spare mount tire kit designed by GM to resolve the movement of the spare tire weight from rear center to left side when trucks were ordered with that option. Not an issue of an old or sagging spring, just adding a leveling effect to counter the weight movement. Mickey's truck didn't originally come with the side mount spare fender. Something he switched to in his build and the spacer was the final piece to finishing the relocation puzzle.
