We have a chicken that is a couple years old who can barely walk or stand. We have to get her out of the coop every morning and place her in the pen. She can grip the perch with her feet and her legs seem like they have normal warmth to them, she can also move/walk a little but it is hard for her to do and she has to flap her wings to do so. When she stands her tail is down in the roachback position. She eats and drinks normally and has also been vaccinated against Mareks disease. We have absolutely no idea what is wrong with her and she doesn't seem to be in pain but it's hard to tell. Any help would be greatly appreciated.