New member with question

Howdy!
I am a new member, and I have been raising 4 sweet chickens for a year now. I enjoy it a lot! My pet Barred Rock,( Dolly) hasn't been feeling too well for the last 2 months. Below her vent is swollen and bald, but it is just starting to grow new feathers. It used to be red, then pink, and now it is finally turning back to the normal tan color, after I put Vaseline, Poultry Aid (on the sores where the other hens were pecking at it.) Below the vent, there is a big, swollen area. Our girls are free-range for 3 hours a day.
She is not egg bound, because this has been going on for at least 2 months. I keep her seperated from our other VERY three healthy hens, because they seem obsessed with pecking at her bald, swollen spot The water bowl is always sparkly clean, and feed too.)

Thank ya'll for helping me!

 

Thanks, guys!