I would simply replace the pto shaft itself. That is one reason I have not gone with a quick hitch, I would probably need to replace the pto shafts on all my implements as well.
Taking a look at the extender, seems sturdy enough, but again, I see it as a point of failure. For me, I wouldn't go with this option. It maybe cheaper than replacing the entire shaft, but not worth the risk. I wouldn't do it.