I'm with the above on the wheel balance but if you want to check the tires having an internal fault, just run up the pressures on all four to 50 lb. and drive it at or above 50 mph on a smooth stretch of road. If the vibration goes away, drop the pressure to normal on one tire at a time and run it back up to 50 mph each time. When the vibration re-appears, that's the faulty tire. If you want to get the driveshaft checked, it's a pretty quick, inexpensive and simple process. Be sure to mark the orientation on the yokes at both ends so you'll be able to put it back as it was. Take the driveshaft to your local driveline shop. They'll mount it up, spin it and check the balance, usually while you wait.