Severe vibrations at specofic RPM when under load

This is very similar to another question on this forum Aggressive car vibration at 1700-1800 rpm. My vehicle is a 2011 VW Amarok 4cyl diesel manual. The vibrations only occur in gears 4,5 and 6 and only at 1800-1950 rpm when there is significant load on the engine, like climbing, accelerating hard or towing. It is more prone to vibrate in the higher gears. Normal sedate driving does not see the vibrations.

The engine is tuned for power and when that was done is when I first noticed the problem. Thinking it was the dual mass flywheel, I had that and the clutch replaced (at great cost) but the problem persists. I now avoid the problem by dropping gears when I need to push harder but it should be able to pull through that rev range. The best suggestion I’ve had so far is a sticky joint or imbalance in the drive shaft. Any ideas for a way to clearly diagnose would be appreciated before I spend more money on what could be a guess.