I would not know about the transmission question, sorry. Another thot tho, have you verified that you have the correct prop on there and the vibration you are feeling is not cavitation? Perhaps with adding all the ballast, the angle changes and causes cavitation (I know its a stretch)....have you tried changing angle by adding bow weight just to test that theory? I am probably way off base here, just something that came to mind....if you have had boat for a while, were running same ballast and never had an issue before, then my idea would be wrong.