discovey vibration

hi all just bought a discovery 300tdi 1995 and its great but have noticed at around 50mph i get a vibration when i am accelerating but the funny thing is it dosent happen every time i get to 50mph .ive checked the ujs and all seam to have no play what so ever and it has all been poly bushed not long ago so all good there ive looked at the rubber doe nut on the rear prop and it seams to be fine any ideas would be great its doing my head in thanks

Had a similar fault with a 300 RR Classic and did all the checks you have done but everything was ok. One day I was pulling away from traffic lights and depressed the clutch pedal to change from 3rd to 4th gear and got a really weird kick back from the pedal - bit like abs kick back on a brake pedal. Anyway, shortly after that I noticed a vibration through the car on accelaration, and the clutch would slip occasionally, particularly on uphill pulls. Oh well, time for a new clutch. When I removed the old one, I found it was hardly worn at all , but 4 or 5 of the clutch cover fingers were bent and twisted. Genuine Land Rover clutch assembly too!
Anyhoo, replaced it and vibration gone.

Steering damper

check the prop shafts,many times Universal Joint is seazed and you wont know until you thake the prop shaft of