How to loosen up her back end?

i think the best thing for you to do right now is go back to the very basics. get her in a rope halter, or the mildest bit you can get away with, and just ride her at a walk around in an arena for about an hour or so a day. my guess is that she walks a little better when she's on her own, so i think it would be best for you to start re-ociating walking normally with riding. she probably just needs time to adjust. when you are riding her at the walk, do alot of stretching and bending, to rebalance and supple her. give her time, and i'm sure she will come around.