Lowering my dually?

Changing axle height changes the drive shaft angle between the transfer case or transmission and the axle. This anle will be exaggerated even more once the rear loaded down. The OEM accounts for loaded and unload ride heights when designing the drive line angle if you go too far out of specification, it will cause joint issues. if you look at some suspension lift kits made for trucks, they will actually have a wedge that is inserted between the axle and springs to lessen the joint angle after adding the lift. You can argue that this is more of an issue in the front to due to a shorter run, but it also happens in the rear.

Read up on proper pinion angles and drive line vibration before dismissing what happens when you change ride height.