To me that is neurological lameness rather than injury lameness and my gut feeling would be Marek's despite being vaccinated. The best that you can do is ensure that she gets plenty of good quality food and water. A chicken sling/hammock would help keep her in an upright position with little food and water pots incorporated into it, so that she is supported in a position where she can feed and drink herself. She would need to be in a cage so that the others don't steal her food.I would also give her some poultry vitamins to support her immune system.
