Probably what happened is the PTO shaft is just a little short for the tractor. When being used with a rotary cutter and cutting tough material the shafts are not overlapping enough and the inner shaft twisted inside the outer. They are now stuck together. Use a pull along and pull them apart carefully. Remove the outer plastic safety shield, the proper way not with a utility knife, reinsert the inner shaft in the outer and place on a vise or other flat hard surface and tap on the outer shaft half where binding and reshape until the shaft will collapse and extend. You may want to view this video and then measure your PTO shaft using the procedure described in the video and see if the shaft is to short. http://www.youtube.com/watch?v=0vBFJcvs5SE