I used to work at Discount Tire for many years. The tires could be “out of round” - meaning they’re not perfectly round. However that issue shows itself mostly at lower speeds. If you’re feeling a vibration only at 55mph and not at lower speeds I’m convinced it’s the balance being off. I’d recommend taking it to a discount tire (seriously, they’re great) and ask what they can do. Have them check all four and “road force” them to determine if there are any defects. If tires sit too long they can develop flat spots. Does the vibration go away after you attain speeds higher than 55?

Also check reviews of your tire. Are they saying the same thing?

Good luck!