1992 Land Rover Discovery Vibration

Hi i have a Landrover Discovery 1, and i have a vibration when i am going 80km-100km. the vibration is coming from under the drivers side.I have reconditioned my tailshafts and still have it, when slow down and leave it in 5th gear you can still hear the vibration. When i am traveling at 80km in 4th gear there is no vibration, as soon i change to 5th and excelerate the vibration is there!

Could you please assist????

Many thanks
Terrance