Dodge w100

Hey guys got a 75 dodge Powerwagon club cab 360 married 727 fulltime 4X that has this annoying vibration at 1500+rpm, whether I am in park or driving. At first I looked at my transmission mounts and noticed they were all rotten and destroyed. (Engine mounts are new.) I ordered some Polly transmission mounts and the vibration is still there. Can it be harmonic balancer, flex plate, or torque converter? Truck runs really good and smooth, besides the fact that it vibrates.