There was a problem with the front AC units vibrating on the earlier built units, but they came up with a fix involving rubber mounts.
Your dealer shouldn't have much problem figuring out what's causing the rear AC to vibrate. They'll probably turn it on, get on the roof, and start touching things such as lines, wiring harnesses or brackets to determine what's transferring to the frame.
2013 Palomino Columbus 320RS
2007 Dodge RAM 2500 6.7 Cummins
Reese 15K manual slider
M.I.L. in Florida for good!!