Botulism is another neurological illness that chickens get from eating decayed meat, fish, maggots, or vegetation near ponds, streams, or compost heaps. Here is alink: http://www.thepoultrysite.com/diseaseinfo/19/botulism
There are several illnesses that can cause lameness in chickens, but you may have Mareks disease in your flock unfortunately. How old were the chickens when you got them and at what age did they start with symptoms. Here are some links for reading...