Hi L.. First, I can tell you that this is totally normal. I have 6 children ranging from a married 20 year old daughter, then 4 boys, then a 3 year old girl. Even one of my boys at the age of 12 wouldn't go for days because we were camping and they were vault toilets and not real bathrooms. It is amazing how the mind can block them by fear.
The things I suggest are: First, if she poops in her undies, the natural consequence is to put her back in a diaper. Usually they don't like that. At 2 1/2, they really don't understand and making them feel like they are a bad kid is not beneficial. However, telling them that it is a priviledge to wear panties makes it worth their while when they are ready. Just keep telling her that only big girls that can not poop in their panties get to wear them and that babies that poop in their panties NEED to wear diapers. It is just too hard to clean up. :)
Also, sometimes the "feeling" of peeing is easier to read for them than the "feeling" of poop. Just let her develop a little more and don't worry. My youngest daughter just turned 3 in November and she just two days ago decided it was time for her to use the potty. She still has accidents at night though, but that too will work itself out.
Good luck and Happy Parenting!