I'm with Jerry. Probably a bad casting.
But one other idea I'd like to toss out. Years ago, it seemed that with every bush hog I owned, I could raise the 3-point hitch all the way without the pto shaft coming in contact with the front edge of the mower deck. Now, with every 3-point mower I own, regardless of which tractor it's on, I can't lift the mower the last few inches of hitch travel. The pto shaft hits the deck. If they were raise too far, it SHOULD bend the shaft, but it COULD (and probably does) put the u-joints in bind. How about that? Any clearance issues with regards to the pto shaft? If a situation like that cracked the yoke, the first hard jolt might finish it off.