It sounds like you have discovered the never ending battle for top dog. I also have had some issues with Katie (2 and a half) since we got Max (1 yr). It started when Max got up about 6 months old and has been off and on since then. I had her brother around quite a bit since he is my son's dog and she was always the boss with him. Max has given her a little more resistance as he is marking and trying to be the MAN. She is normally fine but I have had to set her straight on a few occasions. The important thing is to make them remember YOU are the boss and they are the DOG. I had to stop completely giving her greenies...she goes absolutely berserk with them guarding them! I have found that if she acts up, a time out by herself does wonders. Hope you can get it under control. I really don't think rehoming is a good idea IMO our babies love us they are just like kids, sometimes they just need some extra love and guidance.
OH and as for the leg, have they checked for hereditary problems and all? It may just be that she has strained it but only time will tell.