It could be a couple of things.
1) Dual mass flywheel removed and replaced with single mass. The whole point of the dual mass flywheel (DMF) is to take out some of the vibration caused by the diesel's power stroke. Going to a SMF gives you the full diesel experience.
2) Harmonic balancer damaged. It's happened before, but will need to be removed to be ccertain - not a small job. It's not the biggest job in the world - it's nothing like changing spark plugs in a Tesla - but be prepared for some argument from the bolts securing it, I've heard they can be a bit ornery.
3) Drive shaft (usually just a uni). It's odd that it is experienced at those RPM in multiple gears, it's usually a specific road speed that it's most noticeable at, but slide underneath and give your drive shafts a wiggle (who cares, Anthony or Greg, either will do). If they're loose (that's probably Jeff) you'll need to check the unis and maybe replace the loose one.