I had the same thing happen to my post hole digger. The original PTO shaft is 20 years old, worked flawlessly, I left it outside for one summer (it had been outside for 20 years), tried to use it in the fall and the PTO shaft wouldn't telescope.
TSC wanted $400+ for a new one, I found an agriculture supply company online that had one for $60. Put the new one in, was an exact remake of the original one, had it on my tractor for about 3 weeks which both were left outside for those 3 weeks, and the 2nd one seized on me. I had it replaced through that supply company under warranty for free. What I think happened was the first one seized up in the elements. The 2nd one seized up because I may have tweaked it when lifting the auger out of the hole. The 2nd one was well lubricated inside and out so I know it is not due to the elements.
Are either of these scenarios (elements or tweaked) possible in your case?