If you have a driveline vibration and replace any of the rear suspension bushes with poly instead of the originally specified rubber the vibration is transferred to the body shell and you feel it. Rubber is far superior at isolating vibrations.
Also the same happens if you replace the gearbox mount with poly or stiffer rubber.
So if you are unable to find the source of your driveline vibration and have installed poly (rear) suspension/mount bushes, replace them with rubber first to see if the vibration reduces/goes away.
Thanks Pete....The trailing arms are all rubber did replace thrust washers with poly the center drive shaft support is rubber...