I have been fighting a vibration on my 87 420 also, i replaced the trans and motor mounts..front and real disks..center brg and holder, both axles..new tires balanced on the road force balancer..still has the vibration at accel..seems mine comes in at 15 mph and is gone at 20.....i was replacing the above parts anyway but thought i would have gotton to that vibration by now..wonder how you test the diff bushings....guess i am advising you to take it to a tech ..lol..let him do the leg work so you dont just buy parts and install them with no real success..gl