I just took my 2004 X-Type in for this problem. I dropped it off this morning and when I picked it up in the evening, the problem was gone. I asked the service guy what they did to the car and he indicated that they replaced the prop shaft and re-balanced the rear tires. The car is as smooth as can be at all highway speeds. The vibration actually wasn't too bad but now that its gone, the car feels completely different.