hello, thinking could be a tire out of round or unbalanced. uneven wear. check the upper tierods and pinion gaers. i understand that it does this at certain speeds but could be a possability. also when you brake does the truck shimmy or shake? if so the rotors might be warped. just a few things that i have had in the past might help you.
this might help too to swap the tires around and see if that makes any difference. from the front to rear.offset