its like a table with two legs shorter than the other two and it will rock.
If it is square on the bearers and the is no dramatic vibration and noise
have checked all the other points mentioned.
Are you sure there is no flexing of the hull
and the bearers have not been damaged?
If everything that has been mentioned is checked as O.K. It could be the poly mounts
are too hard ,if they shatter and have broken the mounts there is a serious problem
somewhere, beefing up the mounts in strength may not be the answer,they may need
to be softer to absorb the vibration.