Lisa, I read all posts as well, and my first thought was Mareks like a few others questioned. She's the right age, and Mareks can be found everywhere; birds can be carriers of it, and there are many forms. Unfortunately, the vaccine is only about 80% effective against the virus (I could be wrong on my percentage).
If your girl has the use of ONE of her legs but not the other, I would suspect Mareks, and I would encourage you to contact a member called horsejody - she has dealt with Mareks and knows a lot about it. Also, she's had a few that she nursed back to health...it takes a LONG time, but those that do live end up doing okay. She's a really nice person, too!
Whatever it is, I wish you the VERY best of luck!