Call your local university and ask for the agricultural department. Or if you have a university extension office, call them. Or call the health department and ask for a number of a lab that does necropsies on farm animals. Refrigerate or place the body in ice in a cooler until you can deliver it...
It's so hard to diagnose chickens unless you're an avian vet and you know what you're looking for.
I can tell you, though, the symptoms you describe sound very serious.
You might do what I do, just start her on antibiotics on the off chance that she has an infection. I use fish amoxicillin...