I bought 06 Ram in Feb 2011..2 Weeks later I have this Harmonic Vibration in the rear end @ 30 to 40 mph..Accelerating, Coasting, Braking hell it didnt matter what your where doing it was there..I checked u joints,pulled drive shaft ran a level lazer down it to see if was torqued.Drop the rear diff pan check bearings all clean I didnt pull the axles..I suspected the play at the out put shaft on the trans but my trans guy says thats normal.Very little play..So now im dumb founded.So I buy new tires and rims because I cant stand the 20 inch rims and the rubber is exhausted,Picked up some 17 inch rims and 285 BF Goodrich A/T from Tire rack that worked out perfect.So hear I think I have the problem in the sack put on the new tires...Still there..So now im pist. 2 months go by I was gonna find this problem.Im gonna tear down my rear axle.I could not turn my axles when the truck was jacked up in neutral..I get the tires off tear down the brakes I could not get my passenger side rotor off the dam e brake was dragging..backed off the star on the e brake, slap here together put the hammer down,, rides like a Cadillac. vibrations all gone..So all you folks out there with this weird unexplained vibration if you didt check your rear brakes and e brake give em a check..could be your lucky day!!!.