Was visiting my daughter today and my son-in-law noticed some tire wear on the rear passenger side dually set. I am getting some significant cupping on the inside of the outside tire and the outside ...
I don't rotate my tires either, other than "flipping" the rear outer tires every 30,000 or so because the outside edge wears faster. I also swap the fronts side to side on occasion.
Cupping indicates something's moving that shouldn't be. Have you checked lugnut torque?!?
I would think you'd feel the vibration if things were out of balance.