I would consult with your state vet next week, and find out how much the necropsy would be on a backyard chicken. It may cost less in your state as it does here in OH. They can usually see signs of Mareks, and tell you that it is likely Mareks, but a Mareks test costs extra and takes longer since most send it off to a special lab. A couple of those labs will send you materials to do the testing yourself for under $50. Those include this one, and you have to collect blood for a sample as directed usually from a trimmed toenail:
https://www.vetdna.com/application/forms/aviansubmittalform.pdf This helps you to know if Mareks is asctually a problem or not.
https://www.vetdna.com/application/forms/aviansubmittalform.pdf This helps you to know if Mareks is asctually a problem or not.