What will cause a vibration that seems to be in the drivetrain when driving at 50 mph?

Several things could contribute to this, but you should also answer this question: if you go faster than 50mph, does it go away again? The goal is to find out if it only happens within a certain range of speeds, or rpms, or gears. So, for example, your answer might be that my car only vibrates between 50-58mph, and in 5th gear alone; it does not vibrate at the same speeds in 4th gear. Or another possible answer might be that your car only vibrates when the engine is turning over between 3600 - 4000rpm. If your car is an automatic or you don't have a tachometer, this will make answering this question more difficult. The goal is to be able to give your mechanic more information so that they won't hopefully spend unnecessary time on diagnostic tests. Don't do anything dangerous especially while driving at 50mph+ to try and find this information. A few possible answers: 1) a wheel out of balance; 2) a drive shaft out of balance, perhaps due to wear in one or more of the knuckle joints; 3) worn engine or transmission mounts; 4) a broken or nearly broken piece of body work in the front of the car that is loosening up and vibrating. I would say the U-Joints also make sure you have nothing wrapped around the drive shafts(road debri)