I know this is an old thread, but any updates on the chicken? As I was reading, I couldn't help but remember when I had my bearded dragon and he lived with my mom. My mom was supposed to dust his crickets but she didn't and he had a calcium deficiency. Coupled with an impaction, he couldn't move his back legs. I know that chickens are bred to produce large amounts of eggs over a small period of time and you have to provide them with calcium suppliments or feed the eggs back to the hens to keep their calcium levels up. If the vet can find no breaks or dislocations, could it be that the chicken simply doesn't have strong enough bones to support it's weight?