Thanks to all for the feedback. VERY much appreciated.
The comment about it happening when fully up was more of a 'diagnosis' attempt - I was trying multiple scenarios to try to get the vibration to happen again so I could have a reasonable description for all you knowledgable folks as well as our mechanic.
Aside from the occasional (OK, semi-frequent
) forgetting to lower it before starting the engine and having to do it right then, we wouldn't run it with the lower unit "trailered".
I will post the results - thanks again everyone!