The girls are constantly pecking their rear feathers out!

Are you sure they are picking them out? It looks more like rooster rash.

If you don't have a roo, I would say they are lacking in protein and methione. Try feeding some BOSS. (Black Oil Sunflower Seed)

 

Mine look like that but it is from a rooster. Have you tried Blu-Kote on it? I use that, but make sure you wear gloves or you will have purple hands for a week.

 

Three things to try: give them more room (enlarge coop or run if possible); give them something better to do (like, cut a whole bunch of long grass or roadside weeds [making sure there is nothing drastically toxic in there], or weed the garden, and dump it all into their run for them to eat and scratch through; and try feeding more protein, for instance scraps of meat from the kitchen (freezerburned bits, trimmings, bits scraped off bones after you boil them for soup, etc).

Good luck,

Pat