We had the same issue with our daughter, though at a younger age. She was potty trained at a little over 2.5 but wouldn't poop in the potty for anything! She would never really articulate "why" and I think it was some fear, but also some stubbornness. One of the big issues was that she always pooped standing up and really wasn't comfortable sitting. We tried major and minor bribes and nothing seemed to work. She would cry and scream and have such a fit!
We did similar things as Teresa C mentioned below. First, we said she had to go into the bathroom - that's where people poop. We would let her use the pull-up and then gradually worked on sitting on the potty (in the pull-up.) We didn't want to get rid of the pull-ups completely because then she'd just hold it for days and days which just made life worse for everyone!
We wrapped up about 20 small presents (playdoh, markers, little toys, etc) and every time she went sitting on the potty with the pull-up, she got to pick out a package. She knew that when the presents ran out, it was time to go without the pull-up. She really wanted a "real" camera, so we said she could get one the day she pooped in the potty. When that day came, she was very upset and cried and didn't want to, but I really pushed her and stuck to my guns. After she did it ONE time, that was it. We bought the camera and never had another problem. I kind of wish that I got tougher, sooner.
I've also heard of people using this tactic and then cutting holes in the pull-ups, so they'd wear it and sit on the potty and end up going IN the potty, kind of like what Teresa did with the toilet paper. I have known and heard of lots of kids with this pooping issue!