Since the frame shakes, I would have guessed drive shaft balance (and possibly the yoke / u-joint) - but if it happens in neutral, then I am less apt to pick there.
It is realtively easy to pull the heads and weigh them.

At this point, everything else I can think of is a a lot of work to diagnose. For instance, you could measure the shake on the engine top while it is in the car vs. on a stand ... to isolate/debug the connection between those major system groups.


Robert  
  https://my62mercury.blogspot.com/ 

What are the actual Compression readings? High to low should be within 10% of each other.

  

Happens when stationary and moving 

Have you performed a cranking compression test?

Lorena, Texas (South of Waco)

I have mate, number 2 piston was lower than all the others. 

Actual compression numbers would be helpful.  Performing a leak down test would provide some additional information before actually taking the engine apart.  If the leak down test indicates 5% or less leakage, then cylinder sealing would at least be eliminated from the list of possibilities.  While performing a leak down test, do all the cylinders and not just #2 so you can get a range of values.

Assuming cylinder leakage is within acceptable limits, removing the right bank cylinder head and measuring piston to deck clearances could potentially indicate a short or bent rod or even possibly a piston with a different wrist pin location.  If the deck heights check out okay, then you may be simply dealing with an out of balance rotating assembly with an odd ball weight piston being at the top of the list.

Lorena, Texas (South of Waco)

Thanks Ted, I will keep you posted once have have time to work on her again.