Normally a vibration at speed means one of a few things: A. the prop is bent. B. the prop is loose. C. the shaft may be bent. Do you remember the last time you beached the boat? Maybe there was something under the water that the boat bounced on. How about the last time you trailered the boat? If the trailer was too deep it may have floated over and bent either the shaft or the prop.
If all else has not happened, then check your motor mounts. Look closely at the stains under the mounts. If they line up with the original positions: you are fine. If you see they have moved, then there is no way that your shaft is straight and you need to have a certified tech to take a look at it and maybe make adjustments and reposition the motor and shaft.
Hope that helps.
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