Dear Mommy G,
I have a 3.5 year old who also has issues with pooping in the potty. When we first potty trained her she would pee in the potty just fine, but she would hold her poop for days at a time often dancing around, crying, and holding her privates until she couldn't ignore it anymore. We started giving her Miralax so that she wouldn't go so long between poops. We were very worried for her well being.
We tried reading her books, explaining the reasons for pooping, and even letting her see her same aged cousins use the potty. She STILL tries to hold it and recently she had a trip to the doctor for bleeding because holding it so long caused her to tear her anus. Needless to say it's been a very emotional and difficult battle for us, but I am beginning to see the light at the end of the tunnel. She has gone from holding it in for a week, to 4-5 days, to 3 days, and ever since her visit to the doc we are every 2 days.
She is not my only child and I didn't have this much difficulty potty training the other 2. That being said, I am truly a believer now, that they MUST be mentally ready for it and if your little girl is comfortable with pooping in a pull-up, then for now, let her do so. BUT, give her a time limit, for example buy a large package of pull ups and tell her that once those are all gone, you are not buying any more and she will have to start pooping in the potty like all the other big girls her age.
Of course that is just an example and you are the one that knows your daughter best, so you need to gauge it based on all you know of her. Also, I would recommend discussing the issue with your child's physician to make sure that you are doing everything you possibly can for your child's well being.
I hope that my experience and opinion at least help you a little, even if it's just to know that you are not alone. I wish you the best.