75mph vibration

Well guys, looks like we found our problem. As initially suspected, it's the rear tires. With the car on jacks and in gear, it had a definite vibration/waddle. When I walked behind the car, I could watch the tire shimmy side to side! Back to the tire shop tomorrow when it's not supposed to rain. Hopefully it's just a tire thing and not a bent rim (though I don't know how that could have happened).