I have a 96 b2500 with the exact same problem. right rear vibration starting at 40 mph , as if you were under a bridge with a run-away freight train overhead, and eventually smoothing out at 70-75 mph. After being stone-walled by the !@#$%^&*() dealer 6 times from whom I bought it, I finally said !@# %^, and went through the most simple obvious items first,i.e. tread,balancing, and finally, jacking up the back end, putting it in gear and watch that wheel spin like a paint shaker. That infamous day was March of 2003. I am still driving it. I'm debating whether to go to a boneyard and swap it out, rebuild it, or go to a merciless, motherless, satan's-pride-and-joy,!@#%%&*()+ dealer and "RAM" it right back up his merciless, motherless, coldblooded,satan's-pride-and-joy,where the sun don't shine,looks-just-like-a-salesman's lips, !@#. By the way,it was a ford dealer.