Here are a few links to read on vitamin deficiency and Mareks disease:
http://www.merckmanuals.com/vet/poultry/nutrition_and_management_poultry/vitamin_deficiencies_in_poultry.html
http://extension.unh.edu/resources/files/Resource000791_Rep813.pdf...
She could have curled toe paralysis, which may also cause sitting on hocks or elbows, and is caused by a vitamin B2 or riboflavin deficiency. She also could be showing signs of Mareks disease. I would start her on a good poultry vitamin in her water containing riboflavin, and it would be good...