plhorn, I don't know anything about transmissions, but what has happened to my friend on his boat turned out not to be his transmission, but his shaft key on his prop. The key way that keeps the prop secured on the shaft had worn down. So when you put the boat in gear and had heavy weight or pulling a tuber or boarder the prop would spin on the shaft and feel like it was the tranny. It would lock back into place on and off causing the jerking feel also like the tranny. I would take your prop off and check the key way and see if it is rounded and no longer square. It is the piece that slides between the prop and the shaft rod. It is a small square rod.
I hope it is that and not your tranny. Hope I was of help.