Have a bad vibration when driving the car. It's engine/transmission related not wheels out of balance like.

I have the drive shaft pulled loose from the transmission. When I run the car in neutral the engine shakes very badly.
The particulars:
The engine was professionally balanced by a engine builder including checking the flywheel and it has been driven for several weeks and wasn't vibrating like this,
It's been a quick, but gradual increase of vibration

Question:
Likely culprits?
Clutch/pressure plate?
What about the engine could do this?
It runs strong. Recently did some 1/4 mile runs and it did well. (14.2@104)
There's no knocking type noise from the engine. Harmonic balancer looks good looking at it on the engine.