I think I'm almost there.
I took everything off the tractor, the mule drive, the subframe, the deck, then I put it all back on again.
Except this time I did it the way you are probably supposed to, piece by piece. I used to wrestle the whole deck-subframe assembly under there in one piece. Dont ask me how.
This time I put the subframe in first...and guess what? It still kept sliiding off the tines. But for some reason it suddenly stopped slipping, and I dont know why. It stayed. Even when I put weight on it, it stayed. It must be something to do with how I put it on this time although for the life of me I cant see why it makes a difference. But I came away from it with satisfaction of a job well done, a job done carefully and correctly the way its supposed to be done, instead of my usual haphazard "lets try this" approach. Its a satisfaction I've rarely known. In that moment I knew that no longer would I do stupid, dangerous things, I would be careful and make sure to do things right.
Then I hooked up my portable jump starter, fireworks flew and I melted the lead + terminal off my battery. I still cant cut my lawn.