I also hope it is internal laying or egg peritonitis instead or Mareks. Here are some good articles to read:
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx
http://scoopfromthecoop.nutrenaworld.com/tag/laying-issues/...
If you live in the US, you can try to get a necropsy done on her if she dies, to find out what was wrong. The body would need to be refrigerated in a plastic bag, but not frozen. Do a search for "chicken necropsy" and look for Metzger Farms, and also look for your state Dept. of Agriculture...
She could be suffering from a reproductive disease such as internal laying or egg yolk peritonitis, but she has so many symptoms of wasting, sitting on her hocks, and neck symptoms, that she may have something neurological such as Mareks or vitamin deficiency, too. I would give her vitamins in...