I would separate her from the others, like in a dog crate until she improves. I would also give her vitamins, maybe polyvisol or or something to add to her water. Maybe somebody else could help with the dosage. Maybe she has a diet deficiency or had a mild injury. I asked the questions I did because sometimes Marek's disease starts with a limping chicken (usually, but not always, starts in the left leg). I would keep her separate because the others will eventually pick on her, and keeping her in a small cage can imobilize her and help with healing. Just make sure her food and water are in easy reach so she can rest and heal. Keep us posted.
Jody