I lost my 5th gear, driving 70 with the cruise control on, no load when the truck had less than 100k on it. I had not started towing commercially at that time and the heaviest trailer I owned was about 10,000 pounds. I had not done a thing to the engine because it was still under warranty. Fast forward to the 11 following transmissions. None of them had a fifth gear come off, all had a fully splined shaft.
Your research is faulty and nothing you posted is true other than the design problem with the stock shaft. I doubt anyone can say for certain what the cause of the nut backing off is.
A hackneyed response.
Yet you post nonsense.