Buddy Mayeux

I just purchased a 2014 GMC Sierra a few months ago. I did not notice the vibrations before I purchased the truck. The vibrations seems to be coming from under the pickup and towards the back. My dealership has worked with me and changed the drive shaft, put new tires and balanced them, he even offered me another truck, but when I test drove three other pickups, they all vibrated, some worse than mine.. It vibrates whether I am going 25mph or 70 mph. I read on some forums others having the same problem. It is very aggravating. I have been buying Chevrolet Pickups for 20 years and never had this problem before. I even went to a front end alignment shop and the repair man took it out for a ride and said it was probably the rear axle. The dealership says they can't check the rear axle until they can get approval from GMC. I need some help.