Vibration can be caused by a number of things. One point is that normal cruising RPM should be about 2200 RPM, with occasional runs at higher revs. Above 2500 the engine is working hard.
Some potential causes of excessive vibration can be the motor mounts, cutless bearing, strut, the prop or prop shaft, the coupling, and I'm sure other culprits that aren't occurring to me right now.
My point is that I would not look first at the transmission if vibration is an issue, particularly if it only occurs at high revs.
To check the motor mounts, take the cover off the motor when you're out motoring around. Have the person driving goose the throttle, in your case, go from 2200 to above 2500 if that's the range that's causing a problem. Does the motor lurch (a lot)? If so, suspect the mounts.
The prop, cutless bearing and strut can be checked in the water. There should be zero play in the strut. To check the cutless bearing, grab the shaft forward of the strut and shake it. It's easier to see on land, but if there is play in the bearing, you should be able to notice it. Obviously, the boat will need to be hauled to fix either of these issues.
Anything relating to the prop can be handled with the boat in the water by a decent professional diver. A lot of growth on the blades of the prop can cause a great deal of vibration. I saw this in action after having my boat in Florida for a few weeks this summer. I hopped in and scraped off the barnacles with a paint scraper and was good to go. Also, check to see if the blades are loose. Another issue is that the prop itself can be loose on the shaft. Grasp the prop's hub and give it a shake. There should be no play.
You can also check the prop shaft. Use calipers and turn it by hand to see if it's out of round. Also check the coupling that attaches the prop to the transmission.
Also make sure your motor is tuned up. If it's running roughly, something that often gets worse as you add revs, then that may be mistaken for a vibration.
