How was your truck dropped before.. hangers? You checked to see if the bolts are torqued? One may be slightly banging front then back if the bolts are a little smaller than the holes.. check around the hanger to see if there is indication of one moving.
may be a shot in the dark but hey, we're brainstorming, lol.

 

the original drop was 2" leafs and 2" shackles. New drop is 6" flip and notch, with air helper bags in back which brings it to about a 5-5.5" drop in back.

 

When they get the pinion angle adjusted and set exactly where they want it WITH THE WEIGHT ON THE SUSPENSION make sure they tack the perch to the housing

 

why not just get under it and look for something loose or that may be contacting where it shouldnt be.... before taking it to the dealership

you can wiggle the driveshaft and see if the U joints are shot... and chances are if they were THAT far gone, you'd be getting a vibration out of them too.

if it didnt do it before, and it is now, my vote goes to something hitting where it didnt before you went lower... am I missing something here??