Steering Wheel Vibrate problems of the 2002 Land Rover Discovery Ii

I have owned my 2002 Land Rover discovery se for 2 years. About 8-9 months ago, I noticed that at certain speeds, my steering wheel will shake back and forth, or shimmy, sometimes somewhat violently. I have taken it to aristocrat motors, the only service center in my area for Land Rovers, and have described the problem three times. The first time they said they checked the tires and they were fine. That was it. The second time, I called and asked why it was doing it, and was told by the service manager that that's just what Land Rovers do and there was nothing they could do about it. The third time, I was told the wheels aren't unbalanced and that's the last they said about it. I have a fourth appointment to take it in for the same problem and expect them to fix it or I feel they should be held liable if it breaks later after the warranty has expired.