[GMMerlin]

Vibrations are not that difficult to diagnose, it just takes some math and the right tool.
Personally, I like doing vibration diagnosis, not many techs do or are proficient at it so I have made it my nitch around the shop.
The problem is they are not trying to screw you; they just don’t have the tools or experience to properly diagnose a vibration concern.
Every GM dealer was sent an EVA (electronic vibration analyzer). This is a tool that will display the order of vibrations (order is the # of shakes per revolution of a component or component group).
Also in GM Service Information there is a vibration diagnosis table and worksheet that will assist in making the determination on what order and component group are causing the vibration.
There are some particular road tests that need to be performed to determine if your vibration is related to 1 of 2 scenarios (Vehicle speed related vibration or an Engine speed related vibration)
Certain conditions cause certain order vibrations that can be directly linked to a component group (operational characteristic of a component will cause a certain order vibration)
One thing I would definitely look at is the mounts. If they are damaged, that could be the cause of your vibration. Something could have caused them to fail or they have just disintegrated over time.
But a proper roadtest and inspection will uncover the source of your vibration.