2014GMCSIERRA

I just joined the community after reading about all the vibration issues with the 2014 trucks. I also have a strange vibration in mine but not like a lot have described (at speeds 70-80mph). Mine shutters very hard when slowing down starting at about 40mph all the way to 5mph and not like a brake pulse or tires that u would feel while constantly braking or driving. I took it to my dealer that kept it for about a week and they came to the conclusion that it needed a drive shaft. So they ordered one and put it in and there is no difference. It still does it and it's getting worse, so bad that it's making things under the dash rattle. Now I did have a pair of leveling shackles on it but was told by my dealer that that's what was causing it so I took them off and it's still the same. I'm trying to find another dealership to go to to get it looked at after the last time I went to my current dealer to get my oil changed and tires rotated and they scratched my bumper and left huge greasy hand prints in and out of the truck. I have realized while slowing down I can put the truck in neutral and it will not vibrate! Hopefully when I find another GOOD dealer they can figure this out. Anyone else having this issue? Mine is a 2014 sierra sle single cab short bed 2wd.