I would sit her down and talk openly and frankly to her, without deamonizing her other parents. She's old enough to really get it. Don't leave her wondering, because not knowing is a lot scarier than understanding.
Ask her how she feels about the current custody arrangemet and tell her what your plans are. Reassure her that daddy loves her so much and tell her that a judge will help figure out what is best in this situation. I would personally stay away from telling her you're trying to rescue her from a bad situation, because what if you don't win? How horrible would that feel to her to NOT be rescued? Then ask her how she feels about what you have said. It is hard, but please be careful to not ask her how she feels about all the fighting and if it makes her feel bad. She'll know what you want to hear and even if it's just the simple truth, expressing her unhappiness to you is asking her to betray her father's love and risk making him really mad at you (she'll want to protect you) and at her, making more fights happen in the house.
I want to add that I came from a family that fought a lot. There was a lot of tension in the house and my mom was very manipultive, controlling and loud when she expressed her unhappiness. My parents didn't like each other much and I prayed for them to divorce to end the unhappiness they caused each other. My brother was a troubled young man, too, and that caused more tension. It was embarrassing at times when people would hear the yelling over the phone. Sometimes the neighbors even commented on it.
I'm OK, though. Through all of it, I have not one ounce of doubt in their love for me and how much they sacrificed for me. It turns out that that's all that really mattered in the end. I hated the fighting and I never ever yell at my kids. I now have a very peaceful house which other people often comment on, but I came from a tension-filled home that was also love filled. Sometimes I didn't verbalize noticing the love when I was young, but I am SO aware of it now and I thank God every single day for giving me my mother and father, and for them being (still!) together. At some point I realized that the unhappiness they caused each other was not my problem -- and that they weren't always unhappy. They loved each other, too. It took me longer than 8 years on earth to figure that out. Yes, I would have wanted to leave when I was small but what a loss that would have been. If you take her away, she'll get less active love from them, won't she? I'm nearly 40 now and I still can't imagine the pain that I will know when I can no longer visit them all the time.
I wish you all the best.