My bars don't rock in the post. I suggest Tightening the screw in the backside of the tensioner a 1/2 turn or more.

The only reason my bars might seem to rock in the post is that the handlebar stem is higher than most bikes and physics simply dictate that if you yank on it the metal will give a little.

IMO, this is something that any high stem will do. So like I said before, I don't try for wheelies unless they are minor and I'm more or less in control and I don't attempt mountain biking.