So here's an update on the U-joint. The U-joints on the mower are external (in yoke) snap rings, but the only available U joints with the same dimensions are internal (on bearing cup) snap rings. I bought one of these and had to do a small amount of grinding on the insides of the yoke to remove rough spots, and the internal snap ring version works just fine. For future reference the part number is a 200-1-675 from Burns Industrial, same u joint that is used on lots of farm truck hoists.
Thanks for all the help- maybe this post will help someone out in the future as well