My daughter was the same. So we took a gradual approach to going poop in the potty. She would tell us she needed to go and ask for a pull up. A friend's pediatrician gave her this idea: We told her she could go in a pull up but she had to stand in the bathroom. Poop belongs in the bathroom, not playroom, kitchen, etc. After a few weeks of that, we told her she cold wear her pull up, but she had to start sitting on the potty while she pooped. That got her used to the feeling of sitting and pooping. All along I had some nice toys on top of the entertainment center (in sight, out of reach) that she could pick from when she pooped in the potty. Then one day, her poop leaked out of the pull up and fell into the potty! I took that occasion to tell her that she's a big girl and her poop is to big for a pull up, next time she'll go in the potty, etc. Well, she didn't poop for 5 days!!! Another friend has a son with constipation issues. So per her advice I got Babylax. It's glycerin in a little squirt bulb. After using that, my daughter was very upset with me, but she couldn't hold it in any longer, put her on the potty, and voila! Poop in potty! After that,no major problems. It had obviously been a mental thing with her, and doing it once, showed her it was ok. Plus the toys helped!
Good luck!