She probably has a vitamin deficiency. Best to isolate her from the others and start her on vitamins, Aviacharge is good. Make sure she has lots of chick grit (finely crushed limestone) crushed tums work for calcium too but I think chick grit is better. Give her whatever she likes to eat (not catfood, but variety, fruits, cereal, chick starter, hard boiled or scrambled eggs, yogurt, cottage cheese. Let her have some sunshine when you can. This is a difficult thing to treat, if that's what it is. I'm not sure that splinting the hocks will make any difference, but maybe you could try with light balsa wood and some kind of soft wrap. We have had two pullets in the past years that had this at different times; some may say that it is Marek's, and it could be, but there are a few conditions that have these symptoms, and Marek's should not be the first thing that comes to mind.