Dealers say things like that when they don't want to fool with a problem. Heavy vibration is not normal..in any vehicle. Try this: put it in 4wd and take it on a straight road. As you are going let off and back on the gas. If the vibration changes or stops sometimes, its that drive line. For most..including mine, the splines on either side of the carrier bearing are so loose the line "whips". When it does it feels like a heavy vibration. In 2wd you can't control it but in 4wd you are linked to the engine and your peddle actions will effect how it centers itself...or not. It may be something else completely but this is one thing you can do to check it out.