If the vibration is only their when surfing or listed then it's not the prop. A damaged prop would show up when you were unweighted (but it might show up at a different speed/RPM combo.)
That being said, it wouldn't hurt to pull it and have a shop inspect/balance it. Cheap way to cross that off the list.
My theory -- the swim platform is either a little loose or just shaking at that speed. Have someone sit on your sun deck with their feet on your swim platform. I bet you a $1 it starts to buzz at that speed.
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.