She may be suffering from a riboflavin (vitaminB2) deficiency. That can cause walking on the hocks and curling under of toes. Poultry vitamins (most brands) contain riboflavin. Yes, there is a Mareks PCR test available from several poultry labs including Texas A&M. It costs around $30 according to their website. However, you would need a vet or someone locally to draw the blood, and place it in the right type of lab tube, plus shipping. I would call them to inquire about the specifics. There are ever changing Mareks virus strains, so there could be false negatives.