chicken won't stand up

[cjeisnaugle]

What's her poop like? Any photos?

I agree, it doesn't look too good. Marek's still comes to mind.

Interesting that feeling under her vent between the legs that she passes gas. I've never had that happen.
Her crop is emptying, no lice/mites. Can you take a fecal float to check for worms and Coccidiosis?
While I don't think either of these things would cause the leg issues, they could be contributors to decline.

Poop looks normal to me