Saving Old Seagulls

I have two Forty Plus Seagulls that vibrate a lot (really a lot) when running over half max RPM in a barrel with the prop off. My other gulls do not do this.

Is this normal?
Will it smooth out on a boat with prop attached?
If not normal is there a cure?

I had these symptoms with my Century too. It was a result of a badly worn drive shaft, the square one. Under load it even jumped out of mesh with the main shaft..

Albert