vibration

got a 53 disco gs manual when i get to 65 - 70 mph in fifth a get a lot of vibration trackin all ok wheel brg ok 2 new tyres... shafts seem ok...anyone help please please.. dosent do it in 4th

try having the wheels balanced

When you say vibration I take it you can feel it through the body of the Disco - rather than just through the steering if wheels are out of balance .
IF the vibration is felt throughout the vehicle then :
check rubber doughnut for splits or other signs of failure and check all the UJ's - I note you say they 'seem' OK but you really need to have them checked properly e.g is there any sign of rusty dust on any of them ?
or is there any sign of play when you try to turn one part while holding the other rigid etc
Sometimes the problem only becomes evident when the shaft is removed .