Limping Hen

I've a black Australorp, about 3 years old. She's been molting for a while, but the problem is that she's a bit lame. I think one of her legs is bothering her, as opposed to both legs. She nestles down and lays quite a bit... even when I've brought out some grains for the hens and scattered them about in a small area. It's almost as if this hen is too tired to stand. She'll eat some grains (has a good appetite) then nestle down, then a bit later will stand again to eat.
There's no bumble foot; no swelling I can see. No pecking from the other two hens. She won't get on the roost at night, but sleeps in a corner on top of the pine shavings.
This hen seems fine otherwise than seeming to rest quite a bit. Even during the day I'll see her eating grass, but lying down to eat instead of standing up.
When I bring treats out, she'll watch to see if there is actually something to be had, rather than run over immediately. This has been going on for about two months now, and doesn't show any sign of getting better.

I purchased some liquid baby vitamins (no iron included), thinking I would add some to their water. Is there a particular amount I should add per gallon of water?

Any ideas for me?

Thanks!