I was never aware that they were back spec'd. I have a 04 Sport Trac, which basically has the same setup. But I don't tow or haul heavy either. I just replaced them with 75w90, but if I knew they were backspaced, I would of pushed for 70w140. Like 01rangerxl said, you will be hard pressed to to find a synthetic 80w90, so I really don't see a problem using 70w90 or 70w140 instead. It would be nice just to keep both axles the same, that what I would do.