The vibration you are having which causes the mirror shake is very likely caused by the drivetrain not springs or spring bushings.
If your truck has a torque tube rear end, the vibration can be coming from the rubber mount at the rear crossmember. In my opinion, that little rubber pad is there to tame vibration and not for supporting the transmission.
Another cause of that kind of vibration could be from deteriorated front and/or rear engine mounts.
Have you tried to swap the front and rear tires to see if it makes a difference?
