I had a Jenneau 54 DS come apart on me in the middle of the pacific once. This is unrelated to your vibration (I'm assuming), but maybe worth a look while you're in this project
What happened was that the stringers that comprise the "box" that is the foundation for the steering
quandrant bracket or "base" - they are not reinforced with any hardware
at their joints...they just glassed these together. It's a design flaw. Once you get into some big waves the flexing of the boat literally broke the seams/joints loose, then causing play in the shaft and the boat started taking on water
. We had to turn around 600 miles offshore
and limp home after jury rigging
some line and a twister to hold the quadrant down.
Jenneau got involved but didn't pay for any of the refit
, even though they admitted it was a faulty design on their part.
So, bring it up to the yard and have a look in there while you have it apart, if you see the same construction fab some kind of reinforcement for this boat.