I`ve chased my tail in the past just like you. 1`st thing I did after a drive and vibration check was, I checked to see at what RPM`S(and compared to mph for reference) the vibration occured NOT just the mph and then went home, put it in neutral and went to the rpm`s(mph) at which the vibrations occured to make sure it wasn`t MOTOR related. More times than not for me it has been engine harmonics not trans,d-shaft etc. If you run solid mounts it makes it even worse.