Highway speeds vibration or harmonic that comes and goes

Hello Folks, I have a 2004 Chev Cavlier with 110,000 miles, wounderful car. At normal highway speeds I have a vibration or harmonic that comes and goes. The vibration or harmonic is like it would be if it was an older rear wheel drive car with a bad u joints?

At normal highway speeds I have a vibration or harmonic that can be felt in the floorboards but not in the steering wheel. If it was an older rear wheel drive car it would be a bad ujoint or out of balance driveshaft. This vibration is not constant it comes and goes. I have new tires that were balanced and the dealer replaced the front wheel bearings, that helped some but the vibration or harmonic is still there. Maybe rear wheel bearings or front ujoints or constant speed joints?

Thank You, Denny