I just bought a 1988 super duty diesel. Most of the time somewhere between 35 and 45 MPH it starts shaking violently. I have tried driving through it and it continues on through at least 60 which is when I let up on it. It doesn't do it everytime but at least 90 percent of the time. I know the steering linkage is tight ball joints are good and steering stabilizer seems good. I'm about to rotate the tires and see if it helps.