I second this theory. Ply separation can cause a tire to become "out of round" causing vibration and/or pulling to the side. Flat spots happen on modern tires but usually round out once the tire warms up. In the old days, nylon ply tires were notorious for flat spots. Sometimes you can feel the ply separation with your hand.Sounds like a tire out of balance or a ply separation on a tire. Next would be to check brakes and wheel bearings but they usually vibrate earlier than 55 mph same with front end suspension parts.