1995 Ford Thunderbird vibration problem

Drive Train Axles Bearings problem
1995 Ford Thunderbird V8 Two Wheel Drive Automatic 100.00 miles

During driving a vibration occurs, I think in the driveline. Most of the time, but not always, it occurs around 2000RPM. It lasts for about 2-3 seconds and then it disappears. If I let go of the pedal, it disappears immidiately. In trying to solve the problem I've had the drive shaft balanced and checked the clearance. After that the problem looks less but did not disappear.