It’s not impossible but my thought is that Marek’s is unlikely in a single chicken this age that’s been raised on your own property.
My daughter and I recently rescued an 8-week-old chick that had bilateral leg paralysis with no obvious injuries. We suspected Marek’s and gave supportive care but the key was that her symptoms didn’t progress over time, and even though it appeared hopeless, 2 months later she quite suddenly recovered.
A problem with diagnosing chickens is that lots of illnesses present with similar symptoms of lethargy, weakness and diarrhea. She’s probably very dehydrated so definitely follow the previous suggestions for electrolytes, vitamins and Nutri-Drench as well as keeping her isolated from the other chickens in case she’s contagious. In my experience sick chickens tend to recover or deteriorate pretty quickly with or in spite of the best care I can give them.
Please update.