Personally, I think I would contact the grandparents with a personal note or even a phone call. Explain to them why you have not contacted them before now, exactly the way you have explained it here. Tell them you miss them, and really would like for them to be a part of your daughter's life if that is what they, too, want. This may be an answer to their prayers.
I have a suspicion they do not feel the same way their son does. They probably were taken by surprise and hurt when you moved,and they may have told you they felt differently, had you given them the chance. As a grandparent myself, I know my heart would be aching for that little one if the same thing happened to me. In fact, I tear up as I write this just thinking about it.
Grandparents are usually more willing to overlook a lot of things in order to be able to see their grandchildren.
If they don't want to see her, then you have lost nothing by seeking them out. It is their loss, and your daughter is better off not knowing someone who is not emotionally capable of loving her back. I am betting that is not the case though.
I am praying this has a happy ending for you. :o)
As far as telling her about her "real" father. She is old enough when she asks. Tell her enough to answer her question, then nothing more. It is easy to overload a kid with big answers when all they wanted was a very specific answer to a very specific question. When she is ready for more info, she will ask more questions. Always speak kindly of her father. Never place blame. If he is not a nice person, she will come to that conclusion all on her own in time. (And probably sooner than you think.) It will be painful for her, but not as painful as her resentment for you if she thinks you were "mean" to him. Kids want to love and believe in their daddies, no matter how good or bad they are, until the daddy lets them down, himself. When that happens, they need to feel they can bring mommy their broken heart without fear of "I told you so".