Chicken Walking Funny (Possibly Egg Yoke Peritonitis?)

The White Rock chicken, with the blue fabric saddle on - is walking a few steps, then dipping down. This is much different then the submissive squat that chickens frequently do - so it is not that. Plus, they usually only do that when I actually enter the run. This chicken's age was said to be 18m (about 6m ago) - so she should be about 2yrs, but could be much older. She had no feathers on her back, and her back was blood red - I was told it was rooster/hen abusive mounting - but who knows. I have 2 other white ones that have similar problems with feathers...but not with this problem. I've only had her about 6m, so the exact history is unknown. I have no rooster, and only 8 hens. 5 were mine from birth -and these mystery 3. I am not very attached to these 3 white chickens with the feather issues, so if it is better to put it down, that is okay to say. I agreed to take them, before seeing the condition they were in - and bringing them back to health has been slow - and not entirely successful - obviously. They all seemed to lay a large egg at least 1 every other day when I first got them. Please give me your diagnosis theories in the comment section....I have never really had a sick chicken before, so - I am new at major issues.