This is from notes Tom, I am not taking credit for it. I got these off a very knowleable guy name Zane. He was a Ford Tractor mechanic for years.
It is possible that the sliding coupling that engauges the shaft to the rear of the trans is worn out.
However the first thing I would check is the end play on the pto shaft.If it will go in and out as much a 1/8"or more, the problem may be that the snapring that holds the pto shaft bearing in it's housing is out of place or the bearing is worn out.If the snapring is out of its groove it may be necessary to replace the pto bearing housing in order to have a good groove to hold the snapring in place.Sometimes the snapring can be knocked out of place while bush hoggine etc and will go back in place and hold.This is if the groove is ok.
The drive coupling can be replaced by removing the tpo shaft and the left side shifter plate.Be sure that you observe the locations of things before you remove them so that you can put it back the same way it came out.