The rear springs on my truck were worn out, it came from the Flint Hills of Kansas, used as a plumbing and heating company truck. I could jump on the rear frame / bumper area and the only movement was from the tires, the spring had such deep steps, they no longer could slide or move.
You might also check the tune up of the engine, it might have weird miss or something causing the motor to vibrate shaking the mirror.
