Jim, what did you do to prevent it from rocking within the receiver? Mine has a little rock, but I never bothered to deal with it.
I added a slimmed 2X4 to the bottom of the top step that rests on the bumper. It is a snug fit but slides on easy enough. I made a clamp like PaulT shows and it is rock solid, but I remove my steps every time I move, so it was just something extra to deal with. The clamp is easy enough to make: a square U bolt from a hardware store, a short section of pipe with two holes cut, and a couple of wing nuts. Sorry I don't have a picture of mine; a friend is using it for towing his ice house.
jim