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Lots of things can cause this, and usually there is a combination of many of the reasons.
1. High psi or over inflation (the opposit will cause this also)
2. Light fornt end relative to the rear axel weight (always loaded?)
3. Factory deliberate right bias on the steering setup (insures that the vehicle will usually veer to the right if driver falls asleep helps Ford in product liabiliy cases (reduces head on collisions))
4. Slight toe- in or toe- out condition ( made worse if the rear is heavier than the front)
5. Defective tire/s (I had one out of round on the new truck) they tried to compensate with a LOT of balance weights) Took a while for a competent tire guy to find it) Talked Ford into TWO free tires!
Find a very good tire tech, (not your typical wheel monkey) and have him check it out.