Q: My 08 scion xb vibrates at 55 mph

You did not specify the transmission you have but the problem sounds like an issue with the constant velocity inner joint. When these joints start to have problems with vibration they are known to exhibit problems under a load and only at certain speeds. Have the axles checked for any visual problems and any carrier bearings if equipped. If no visual problems are found, I’d recommend having the mechanic duplicate the issue and verify the axle that is causing it. You may not be able to isolate the single axle at fault so I’d simply recommend replacing both. If you decide to get this fixed, consider YourMechanic, as a certified technician can diagnose the vibration you’re experiencing and make the necessary repairs to your car.