Ask Smithy Xtra Vibrating Falcon XT

My friend's BA Falcon XT sedan has a persistent vibration starting at around 80-90 km/h, but continues until the vehicle is brought to a stop. In an attempt to get rid of it he has fitted new tyres, balanced them, and machined the brake rotors; he has even swapped wheels and tyres from another car, and replaced the left front wheel bearing, all without any success. A dealer suggested he replace the right front wheel bearing, which he did, and he also machined the brake rotors again. He has had the car checked over by the local Ford mechanic, plus a wheel and tyre retailer, but hasn't been able to find the cause of the problem. Has anyone else had this problem with this model Falcon?