I've got a '99 F-350 doing the same thing. I took it to my cousin's garage to check it out. What we found was the hub bearing was bad and needed replaced. The prob with mine is they changed the hub bearing on 03/22/1999 (of course mine was made 03/17/1999)... I just bought the conversion kit to change the hub from the old sytle (my current) to the new style (03/22 and newer). Cost me 280.00 for a pair of hubs and 140.00 for a pair of rotors. The only reason they were so expensive was I have 4 wheel abs (rear only ABS is alot cheaper)SO I HAVE AN 02 F350 AND WHEN I AM DRIVING DOWN THE ROAD WITH THE HUBS ON AUTO AND ITS IN 2WD I HIT A BUMP AND THE FRONT END STARTS VIBRATING AND WHEN I KICK IT INTO 4WD IT STOPS THEN I PUT IT BACK IN 2WD AND HIT THE RIGHT BUMP AGIAN IT STARTS VIBERATING AGAIN, PLEASE HELP ME OUT AND TELL ME WHAT IT IS, AND ALSO WHEN I HAVE THE HUBS LOCKED IN IT IS FINE.:dunno:
To test the hub bearing, we jacked up the truck (1 side at a time for the test), placed a jack stand under it, with the tire lifted off the ground, place a digging iron or pipe (something that won't give) and see if you can move the tire/rotor in and out. If so, bad hub bearing.