You might get a number of replies that say to take away the pull-ups, but I have another suggestion. I had one child that was doing the withholding thing for poop, not pee, so it might be different, but I started off letting him have the pull-up when he asked for it. I was happy that he was recognizing when he needed to go and worked on that before pushing where he needed to go.
So you could put her in underwear and have her ask for the pull-up when she has to go. But also put her in the bathroom so she learns that it is the place to go potty and then put the pull-up on her in the bathroom. You can make it an extra long effort to clean her up afterwards if you want and calmly say, "When you are ready, we can go in the potty and then it will go much faster."
As a second step, you can also have her sit on the potty in the pull-up. So she still has the comfort of (or control over) the pull-up, but is learning that she sits on the potty to go.
Lastly you can cut a hole in the pull-up and have her sit on the potty if necessary as a last step before ditching the pull-ups.
Also, if she doesn't see you go, have her go with you to the bathroom so she sees that it's normal. Have her hand you the toilet paper if possible and let her flush.
It took us a while to get through this stage, but it was less stressful all around.