I've asked this question a couple of times. There is some thought that the cross member for the factory installed class III hitch is identical, so if you can find someone to cutout the class II receiver and weld in class III tube, that may do the trick. Again, this has been a thought. I've asked if anyone knows for sure if the crossmember are the same, I couldn't get a definite answer.
If you find out any more, I would love to hear.
Thanks