First thing: While on the trailer soap up the shaft bearing and stuff a garden hose in the strainer, fire it up and put it in gear.
Now go look at the drive line.
Is the shaft whipping?
Does it look like the hub is wobbling on the shaft?
If the shaft is whipping, send it back and try again.
If the hub has a pronounced wobble it may be miss-bored. try another wheel after making sure the key isn't binding.
If the hub has a slight wobble or you see no discernible anomolies try lapping the hub to the shaft taper.
First make damn sure the key is not binding the prop hub. Install the prop with no key and mark the shaft with a sharp pencil against the leading edge of the hub. This mark must re-align when the key is installed.
If that is Ok it's time to lap the taper:
Cheep and easy and has fixed more vibration issues on big dollar yachts than anything else in my career.
Your local auto parts house ( you may need a real parts house, not autozone) will have Clover valve grinding compound. Buy it in coarse.
Slather the compound on the inner hub and shaft taper, place the wheel on the shaft with no key. Lock down the shaft and spin the prop until you are bored of it. (1 beer)
Pull it off and clean it up, Any high spots will be shiny. Rinse and repeat until no high spots remain. (2 beers)
All other things being equal the boat will run as smooth as hot love.
For what its worth, all the best.