17SierraDenali

Hello Everyone, I recently traded in my 2008 Sierra Denali for a new 2017 Sierra Denali which I’ve had for about two weeks now. The truck is a 4WD short bed with the 6.2L/8 speed and has factory 22” wheels with the Bridgestone Dueler AT tires on it. I have been noticing a vibration/shudder/rough feel which is felt mainly when decelerating under 40 mph all the way down to a stop (it is not brake related as this happens with or without braking). It feels like it is coming from the rear of the truck and can be felt throughout the cab. When I shift the truck into neutral when it's doing this it's perfectly smooth. I did mention this to the dealer and they wanted me to drive it for at least 500 miles and that is where I’m at currently. Just trying to get an idea of what may be going on here and causing this. Any input would be greatly appreciated. Thanks