Ford Falcon: Tailshaft shuddering

I recently purchased a new FG XR6 Falcon cab-chassis. It looks great, is comfortable to sit in, has all the mod cons one could ask for in a ute, but that's where the good bits stop. On the first day I noticed a vibration at about 75-80 km/h, which felt as though there was a problem with the transmission or tailshaft. The service manager at my dealer drove it and agreed there was a problem, but told me it was a common problem and Ford say it can't be rectified. Have you heard of the problem?