sounds like you will need to pickup an entire new "pto shaft" or you might be able to from U joint that connects to rear of tractor. to U joint that connects to gear box on PHD (post hole digger). you might get lucky to find a square shape, shaft end, but when i went looking a year or 2 ago. i did not come up with anything cheaply. and for a few pennies more, could of had a new PTO shaft, U joints, and ends. with protect shields. i saw new vs just the replacement portion. due to i had little luck finding square shape tube ends, cheaply.
the "auger bit" and how it connects to the gear box. should have a "shear bolt" so if ya get bit stuck. or strike something hard. the twisting action cuts the "shear bolt" up. but it requires correct "grade" grade = how soft or strong the metal is of the bolt. to strong of a or higher grade of bolt. and you end up damaging gear box. or internals of the tractor. to soft or low of a grade and you are constantly replacing shear bolts as you dig holes.
if ya going with new pto shaft. you might want to double check if it has a "shear bolt or not" or perhaps pay a bit more for a "slip clutch" slip clutches are more widely seen on bush hogs / rotatory cutters, and 3pt hitch tillers. and not see as much on PHD (post hole diggers)