when i shut my truck of it shakes really really rough it never used to do this. when i start the truck its fine it runs great i have no clue what it is. it almost feel like a motor mount is missing it shakes so hard when turned off

 

i think you might have an injector issue or something, it sounds to me like some of the injectors are still getting fuel and it's trying to stay running, the cummins shakes when it is turned off but that is a whole different beast, the duramax should be essentially like turning off a gasoline engine in a sense, relatively smooth, check the motor mounts just to be sure, but you would probably notice that while driving too via vibrations or noises heard in the cab

 

Are you getting any codes?