Little late.. I had a '03 ram with the rear disc brakes/shoes for parking brakes. I had the same issue with the rear feeling like it was shaking all over that got worse the longer you drove and harder you braked.. turned out to be a sticking caliper. Don't know if you have the same setup but something to have him check if you do Sometimes I swore the rear end was going to fall out that thing
I posted on another one of your threads about the truck how my brothers Dodge had a somewhat similar issue and it turned out to be a common issue. I think it is termed the Dodge Truck Death Wobble http://www.tire-review-expert.com/dodge-truck-death-wobble-fix/ As to whether or not this is the issue, you should still be aware of it and perhaps address the issue before it actually happens.
Mine had that too.. terrifying.. happened whenever you hit sharp bumps on the highway. Only way to get it to stop shaking violently was foot off the gas and let it coast out of it. Brakes made it worse. When that starts though you know its coming from the front.. you feel it through the firewall, your white knuckles on the steering wheel and the pucker factor LOL
