Does she have other neurological symptoms? Mareks is kind of the thing that comes to mind when they can't walk or they are off-balance, but you can't be sure without testing. If she dies (and I hope not) she probably should have a necropsy done by your state lab for poultry. Sometimes there is no cost, and some states charge something. You need to find out about it before it happens because the body has to be refrigerated to preserve it, and they can give you instructions.