What is the horse doing? If it's kicking or being stubborn/disrespectful the other's suggestions will work. However, depending on how old the horse is, what exactly it's doing, or even how long the problem has been going on it may be a pain issue. You say she's a rescue horse she could have some back issues related to her previous circumstances.
My old mare has a REALLY difficult time picking her back legs up and keeping them up. She is arthritic and sore in her back end, so when I'm trimming her hooves I work quickly and give her lots of rest time. Before I realized what was going on there was a farrier out trimming her and he got "rough" with her because he said she was being disrespectful/rude. It didn't help the problem and I ended up having someone else come out. After working with her the second guy told me that in his opinion it had nothing to do with attitude, it was actually pain. What seemed like her being disrespectful (pulling leg way up, almost falling on you, pulling leg away) was actually her trying to balance herself. So when he asked for her leg he let her pull it up and he allowed her to balance herself without interference on his part. Once she was balanced he slowly moved her leg into position. No more problems. She still needed more frequent breaks, but he gave them to her and it worked out fine.
Just a thought.