I've identified that the rattle is coming from the hinge for the tailgate. It only makes the rattle/vibration at mid range RPMs. If I jam a piece of flexible rubber mat into the jam of the tailgate it stops the vibration form occurring. A permanent fix would be a rubber or plastic bushing inside the hinge to prevent it from happening. I'll inquire with the dealer and see what they want to do for a fix. Thanks for the responses.
Scott