There was 10 years ago or something like that a guy that had a RAM web site and this mod was documented on it.
here is a quote regarding doing this mod.
"I took the spacers out last Saturday took me just over 2hrs. Moved the spacers from under the spring to above the spring reused the old u bolts and spring center pins. Dropped the rear bumper about 1 1/2 inches"
NO one back then complained about drive line vibrations after the mod. Chris