Bad Vibrations

Howdy all, Something to ponder.....
I have a harsh vibration under medium to hard acceleration at 55km/h (31MPH) and it comes in again at 100Km/h (62MPH). SO far we have tried this to no avail:
Remove rear prop shaft - problem stays
Remove front shaft - problem gone
Install brand new front shaft - no difference
Dissasemble and check centre diff - all just fine
Dissasemble front diff - all fine
Checked both CVs and front wheel bearings - all fine
Changed the wheels for some from another vehicle - no difference
Put the truck on a hoist and run it up to 55 - transfer case starts shaking - engage diff lock and the problem is almost gone.
The problem appears at the same speed in low or high range which to my mind rules out the gearbox. It must be from the transfer case on.
Where to next??
Any clues?? It is a '98 300Tdi with 80000km.

OK all! Here's the latest!

I MADE them change the front output bearing today. It has made an 80-90% difference!!!! BUT, the vibration is just still there at the same speed but only just. Now, i'm going to have the driveshaft ballanced tomorrow to see if that is the problem and if it ain't, well I'm not sure what to do next as this small problem could stuff another bearing in not too many miles and we end up back at square 1.


I'll keep y'all posted....

Hi all,
Today's information:
ANOTHER NEW donut - The old 1 had torn!
Ballanced both driveshafts - they were both out but by less than 1oz.
All this has made a difference - quite markedly.
Smooth now - EXCEPT fot the 55km/h point. I'm beginning to think now we have a bush issue, maybe the locator arm bush?
Thoughts???

Howdy,
Hmm, well wheel ballance wed arvo if I can. Do y'all reckon the spigot bush should be changed with the donut? This is the bush inside the rear shaft that the spigot sticking out of the diff goes into?