Good news -- although we never figured out what was wrong with her, our limping chicken is no longer limping. A few weeks ago, she started to seem to be moving around better. Now, she runs around as well as the others. In fact, I can't even tell which one was the limper! I wish I could say I did something that worked, but whatever her ailment was just resolved itself.
That is great news. It may have been an injury that finally healed. Thanks for the update.