1995 Buick Lesabre vibration

Shakes or Wobbles problem
1995 Buick Lesabre 6 cyl Front Wheel Drive 140350 miles

vehicle has a vibration when it changing gears at about 40 mph when the speed gets up to about 50mph the vibration stops could the front axles need to be replace or could it be something else, the trannie sounds ok whenit gets up to 50 mph thanks in advance jpm