Transmission Possible

I MIGHT be able to shed some light on your reader's vibration problem with his Falcon. I have a driveline vibration when driving my AU Falcon between 78km/h and 85km/h and the tailshaft has also been replaced without improvement. I found that by changing into a lower gear, but not changing the road speed, the harmonics of the vibration were removed. So his problem might be a transmission fault like mine, not the driveline as Ford is telling him. With the warranty on my car expired, my only option is an expensive transmission replacement.