go around back of the tractor with engine off. grab the lil pto stub and tug/push on it. very likely it has some in / out motion. if it does, etiher: the bearing is worn, allowing play, or the wear ring has slipped, allowing play, or a split ring has popped out and allowg clearances.. or of course I guess if the bearing arrier housing is broke.. that would do it to.
it's easy to pull and R&R the pto shaft.. I've don't just about every one on my pre 65 tractors .. in fact.. I need to do the one on my 3000 as well as it just developed a lil play.
the play allows the shaft to slide in and out and that walks the dog clutch out of engagement, and then the splines can chew up.. untill you ge the parts or assy to fix it ( seal, bearing, wear ring ), at least bunji the pto engage lever into full engaged, that way the splined sliding coupler can't back off the end of the shaft.. even if the shaft is moving for / aft a lil. keeps things from getting chewed up.
ps.. the pto shaft assy is easy to rehad.. just takes a handfull of parts.. ( 30$ or so? ) or about a 120$ new assembly.. etc.
soundguy