C.P.
We had the exact same problem with our son. He would pee like a champ on the potty all the time, but would only poop in a pullup. After talking to every M. I know, we finally just took away the pull ups all together. He didn't poop at all for a day or so, and then he went into the bathroom all by himself and pooped on the potty. And he's been doing so ever since.
Now the down side to this is that he wakes up in bed in the mornings wet (sometimes even at nap time) but he's getting better and better at staying dry all night. We definitely don't want to take a step backward and put him back in pull ups. We've been pull-up free for about 6 weeks, so it's a process. But we just told him that they no longer made pull ups in his size and that we didn't have any more. He found a few swimmy diapers the first couple of days and pooped in those and then, like I said, he just went into the bathroom on his own and did it.
Good luck!