1997 Land Rover Discovery shaking in all gears!

Hello, I have a P reg 1997 300tdi land rover discovery, has done 105,000 miles. It shudders very violently when driving. This is in 2nd, 3rd, 4th and a little in 5th gear. It runs perfect on idle and in motion at any speed when ever the clutch is depressed. Engine mounts are all intact and when engine is revved when parked everything is as it should be. The shuddering is only when I accelerate with the revs between 2000rpm and about 2500, (I never rev it more than that) in any gear. I have been told it might be the thrust, or release bearing on the clutch or a worn clutch plate, but it changes gear without any effort and smoothly. But when clutch is lifted and throttle applied it feels like its going to shake its self apart. I have crawled underneath and everything seems in place, but there is play, rotational and very slight up and down in front prop, or drive shaft at the transfer box end.
I would appreciate any advise, or known cause of the problem, also if problem is known, a fixture price or any advise would be great.
Please write back. Thank you