[garbubba]

Used to have a few of those inline I/O's. That doesn't sound good, but it may not be a disaster.

Since you only feel the vibration in gear, check for endplay and loosness on the prop shaft. If that all feels good, maybe the vibration is just sympathetic.

Since it's noisy all the time let's assume for now it's in the upper half, the first place I'd suspect is the gimbal, if that joint starts to come apart it does just what you describe. That requires pulling the drive though so it may not be the smart place to start.

You could eliminate the upper gearset just by opening up the cover & see if things look OK there before you pull the drive.

The second suspect place is the bearing between the gimbal & upper gearset.

A length of hose or a stethoscope can usually narrow down where a noise originates.

Checking the gear oil should show if gears are chewed up.