Explorer Sport Trac vibration


My 2002 Sport Trac with 72,000 original owner miles has had an intermittant “seat of the pants” vibration for the past 50,000 miles. No amount of rotatation, balancing or evan a new set of Bridgestone Dueller tires seem to make much differance. It comes in at 62mph. But it comes and goes depending on if I am going around a curve and/or accelerating or not. It seems to smooth out at around 80mph! Recently my Ford mechanic, in desperation, switched all 4 of my mounted tires and wheels with a set from another Sport Trac. They were a different style wheel and Michelin tires. It was smoothe at all speeds. Go figure! Thanks.