Before you take it to any dealer you should ask if they have a tech that has been to the vibration analysis school and if they have the diagnostic tool needed to isolate the problem. Some dealers may have the tool but it's broken or they don't have a tech that is trained in its proper use. You should also have any and all TSBs relating to the issue with you when you see the service manager. If you are so inclined, you could get under the truck and see if there is any play in the pinion bearing/gears by trying to move the driveshaft foreward and rearward for checking the pinion bearing endplay, then block the wheels if the truck is on an incline, put it in neutral and try to turn the driveshaft. If there is more than the slightest movement there is a problem with the gear settings. If you have owned the truck since new you should know if the rear has been apart or not, if you bought the truck as a demonstrator or used unit may have been apart and repaired by a tech that didn't know what he was doing.
Hopefully all this info that you have gleaned here will help you.
Let us know the fix when you get it repaired.