A good vet that specializes in behavior is a start. Also obedience school. Your dog has you very well trained!! What a smart dog!!! Actually I mean that. German Shepherds are incredibly intelligent. Sometimes they need jobs. Study up on both the breeds in your dog and you'll understand them better. Sounds like your dog has quite a drive. It could be channeled in ways such as agility, search and rescue and obedience competition.
Don't give up!!!! And there are good drugs out there but that's the last resort. Dawg is right. Don't give her what she wants and eventually she'll realize that the yipping is not getting her what she wants.
Crate training is also a good thing. But not as punishment. More as a cool down place.
Some use shock collars. I've never used them. I'm not sure what I think about them. But people that I know that have used them (with restraint) have had good results. There are lot of different opinions on them so I'm just putting that out there. I'm not advocating them.
hang in there!
Mandette