C.M.
I had this same problem when my daughter was almost 3. Dr Greene has some great advice on this topic: http://www.drgreene.com/21_762.html
We started giving her a teaspoon of Benefiber in every cup of milk and probiotics once a day to get her stools soft and regular and then told her she had to poop in the bathroom - she could still use her pullup but she had to stand in the bathroom. After a few days then she had to sit on the toilet still using her pullup and then again after a few days I took away the pullup and she's been using the toilet eversince (this was about 8 months ago)
Good luck, I know how frustrating it can be.