Prop does not spin when shaft is turned....What's broke?

[SuperYooper]

I have the lower unit off my 1971 mecury 650. There is something wrong with the gears as it will not shift into gear correctly. I adjusted the knob so it should be in gear, but the prop does not spin when I twist the drive shaft by hand. When I turn the prop by hand it moves in both directions with a little friction, but doesn't stop going one way as it should. When I put it in neutral, the prop spins freely as it should.

What is busted?