Here are some thoughts i had:
Shaft is bent slightly, and only gives a problem under heavy load.
A bent shaft would cause problems, particularly vibration, at all
speeds and under all
loads. You can check run-out with a dial indicator, but it's very tricky to do, because any attempt to grab and turn the shaft will tend to deflect it.
Shaft alignment is another story. It could be that. Was shaft alignment checked after the boat was splashed, following the re-powering?
Prop is cavitating.
Prop cavitation doesn't sound like a "scaping" sound, to my ear. I don't know quite how to describe it. Besides: A cavitating prop usually results in a sudden increase in engine RPMs, as if part or all the load had suddenly been removed. Also: You don't experience the thrust you're attempting to apply.