Mine is starting to go also. I don't think that I have 3 mm of play like trantor12020's does (that seems excessive), but mine was diagnosed as a little worn last June when I bought the boat.
I started noticing more vibration at speeds in excess of 2500 rpm (Yanmar 3GM30F), so I dove this weekend and quickly inspected the prop, shaft, and zincs. Prop felt clean, shaft had some hard crusty crud on it that I rubbed off with my leather gloves, and the zincs felt okay and were intact. I didn't have a dive mask, so my vision was somewhat murky. Motor feels a smoother now up to about 2900 rpm, then vibration begins again. I plan on replacing the bearing this winter anyway and since I have only about 8 more sails left until the boat is hauled, I am going to baby the tranny/shaft and not run it under load more than 2200 rpm.
So I recommend first taking a swim and cleaning the shaft and prop of any stuff. Even a small amount of debris will throw off the center of balance of the rotating shaft/prop significantly.