Roto, I have been around farm equipment for most of my life. I have never seen what you are looking for although it would be easy enough to make. The hole in the drawbar is oversized for a reason. It allows whatever you are pulling to twist without bending the pin or worse the tongue of the implement or your drawbar as you drive the tractor over rolling terrain or through gullys etc..
I know it makes some noise as you go along, but believe me, the noise doesn't cost as much as new parts to replace broken parts. /forums/images/graemlins/frown.gif
Anyway, my advice would be to put up with the noise (sorry) and you'll never have any problems from binding that drawbar in something.
Good luck /forums/images/graemlins/smile.gif