When you say it's 3-4" too short, have you operated the cutter using it?
As far as using the PTO shaft from the post hole digger, shouldn't be a problem unless it's too long. You really don't want the PTO shaft bottoming out. That can damage the cutter gear box and the tractor PTO.
New shafts aren't that expensive...Northern Tool has them for about $100 or so. Why not just buy another one?