Defender 200 TDI with vibration in 5th gear

Hi.
My defender has developed a nasty vibration when i slip it in to 5th. All other gears appear to be fine and the truck drives well. if i go over 40 and move to 5th it starts to vibrate so bad , it feels like the front wheels are going to come off. if i dip the clutch , change down and rev it away in 4th , the vibration goes completely.
i have put the trans box in neutral and and going through the gears is fine in 1 to 4 but it grumbles in 5th.
any ideas

Ive just drained the gearbox. the oil looks okay. the filter had a very small amount of non metallic sludge on it. the drain plug magnet had nothing. i poked a rod magnet in the drains and nothing came off.
i drained the box and got about 2 litres or just over of oil. i filtered the oil and it had a fair bit of sludge in it. it looks like fine carbon flake.