Hi, I just popped in and found your thread. Your hen's case sounds very familiar to me. We have had several 2 year old birds over the last year that have had similar symptoms as yours does. I've strongly suspected Marek's all along, but I've not had any lab tests and all of our birds were vaccinated for Marek's at hatching, so it's still a mystery.
A couple of our chickens effected, died rather quickly, but I have an Egyptian Fayoumi hen that has been quite the warrior and pulled through. It's been a long process, which started last March. She started out limping and listing, to then, not walking at all. Egg production also stopped. I moved her to a small brooder pen that, luckily, I had just built. She had her own, little house inside and a nice view of the other chickens, so she didn't feel so isolated. For the first, 5-7 weeks she was immobile, but slowly, she started painfully gimping and scooting around and then hopping. I moved her back into the main coop with the other hens at the beginning of summer. Her little house came with her, so that she had a place that she could hide out when things got rough. By the end of summer, the house was removed. At that point, she could get around pretty good, but still had a limp and looked as if she was still in pain at times. The other hens had stopped picking on her and she was holding her own. It's now January and you can hardly tell that she was ever sick...there's just a slight, occasional limp and she is well accepted by the rest of the flock.
I guess that I'm writing this to tell you that if you have the patience, there is hope for your little hen. Build her a semi-permanent home within your coop. She won't need much space, just some TLC. If she's sick, then the other birds have already been exposed anyway. Don't over-treat her...time and calmness is the best medicine. Good luck.
~C