Hen having trouble walking

Cookie, my 20 month old Speckled Sussex is not looking good. Unlike the rest of the flock, she didn't seem to come out of her fall moult. Her comb is still pale and she just hasn't looked too good.
Now, in the last week she is having trouble walking. She is slow and has to plop down to rest frequently. Her steps are very awkward. And today she was unable to hop the 12" to go out the run gate with the rest of the flock. She clearly wanted to but just wasn't up to the effort, so she turned away and just sat down.
I picked her up 2 days ago to feel all over, hoping that I would find a painful leg or some sort of injury. Nothing. And her vent is clean and doesn't look swollen.
I'm afraid she is taking a slow path to demise. Any suggestions or thoughts on what might be going on?

 

Thanks for your assistance. The feed store where I picked up the chicks claims they were vaccinated for Mareks. I don't know how much she weighed previously, but she certainly felt quite lightweight when I picked her up the other day. I didn't notice any leg swelling, and I poked around her feet. Though I didn't know about Bumblefoot, so I will go check her feet again tomorrow. I am happy to try vitamins and will figure out how to isolate her to make sure she is the one consuming the vitamins. Any particular type of vitamins?
Any other thoughts?