A symptom of Marek's disease is the legs going front/back in opposite directions. Most of the time confirmation is found at necropsy, there is a test that can be done on a live bird, but not many places do it. It just requires a small blood sample. Since you have a vet that will see chickens, I would ask them.
This lab does it:
https://www.vetdna.com/application/forms/aviansubmittalform.pdf
This lab does it:
https://www.vetdna.com/application/forms/aviansubmittalform.pdf