When I got my old post hole digger, Greenline EM5000, It had been setting in an open tool shed for a number of years. The pto shaft was frozen up and the auger was frozen to the output shaft on the digger gearbox.
I took the pto shaft off and turned it upside down so the opening was to the top, I poured a 50/50% mixture of automatic transmission fluid and acetone down into the shaft. After a few days of treatments, I hooked one end with a chain to a tree, the other end I pulled gently with my tractor.
I was able to move the shaft and soon had it apart, cleaned it good and apply anti seize to keep it lubed. Then I took the attachment bolts out of the auger, had applied the 50/50% mixture of automatic transmission fluid and acetone down the shaft. Took the digger out to the garden and started digging holes, dug about three hole and all of a sudden the auger started turning on the shaft. Took it off, cleaned the shaft good and applied a liberal amount of anti seize to the shaft. Old digger works fine now and I could get it apart, if it wanted to.