J.N.
Kids are really good at picking up on attitudes of others. She may be sensing their disapproval and not know how else to respond. It sounds like they don't see her as the amazing and wonderful child you get to be with every day (not saying that they're choosing to see her in a certain way or anything).
Stay with your daughter when you go to visit. Don't leave her there for babysitting until she is comfortable. Be willing to be right there with her and let her just play with you and dad if that is what she wants. Help her greet grandma & grandpa and say good-bye. You can help her wave, and you say "hi grandma! hi grandpa!" in an excited voice. But don't push her to go to them. If they try to hold her or get closer than she is comfortable with, gently remind them that she needs to be the one to make the choice. She needs to feel safe and secure at their house, and part of that is knowing that they won't intrude on her space.