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.