Shaking / Vibration arnd 2000 revs when accelerating uphill

[xpz393]

My 2006 2.0 TDI B6 has developed a problem where it shakes/vibrates under light to moderate acceleration uphill around 2000 revs. Here is what has been checked/changed so far with no effect on the problem:
Clutch and DMF
Rebuilt Turbo
Tyres and wheels checked
Tracking checked (and corrected)
Wheel balancing checked (and corrected)
Diagnostics ran - (nothing found)

Two independent VW specialists and even the VW main dealer have admitted that they’ve no idea what to try next.

Since we bought the car back in 2010, a full service has been carried out without fail every year (and I mean a FULL service), and according to the history, the previous owners kept up with this too. I use decent fuel (Shell/BP) too.

Any thoughts? Googling-around, I’ve been unable to find anyone else with the exact same problem.