Drive shaft?.55 mph or higher the truck wobbles on the floor

This could go on forever but I'll try to make it short. When I drive the truck at speeds appx. 55 mph or higher the truck wobbles on the floor. The steering does not shake. I have had it to the dealer 4 times and this is what they have done so far. They have completed front and rear alignment. 4 brand new tires, total balance of all tires and now they say I need the drive shaft. Thanks.