Hmmm, interestingly enough my neighbor who I "advised" also not to get a second dog (a Boxer) ... he got her anyway and she looks ... just like that???
He already had one dog who he does not ie can't walk, he's on Oxygen and I knew that dog would be a "Backyard" only dog!! But I suppose he ducked your issue as he is retired so he's always home. I visit them on occasion and the dog seems happy enough.
Honestly, you "should have done a "two week shutdown" and the dog should have been "Crate Trained" and taught to settle in the "Crate" or there will be "Consequences!" But before even starting ... I'll let it go and post this instead on the Crate thing as suggested by another member. Not necessarily the way I'd go but couldn't hurt.
The Second Hand Dogs: Playing Crate Games
Moving beyond the "Crate" you "might" be able to overcome "the crap behaviour" by going back to basics. Yes poor breeding, bad gene,s etc etc doesn't matter "now" what's is done if you plan to keep her and if you don't??? What you need to do now ... is "Work with the Dog in Front of You!"
If you do that ... "you" just might be able to overcome the "S/A" without addressing it directly ... I don't know??? But assuming I was going to take a "different and simpler method???"
I'd start by "Walking this dog only" one on one time with you is important! A proper structured walk helps you to build a "Bond of Trust" with a dog.
I don't know if the dog walks well on leash or has people or other dogs "issues" if she does ... no problem I can help with those. Hmmm OK I'll add no dog on the street meet and greets and Dog Parks! You don't want to add to her issues!
And "Sit on the Dog" and "Train the Place Command." Those two are very important and are pretty much SOP for serious trainers that can actually help people with "Serious Issues." They help to train "Calmness into a dog."
Near as I can tell, those two are all explained in this thread. :
Fearful, Anxious or Flat Crazy "The Place CommanD
Aww heck ... one more "baby thread" it's important ... see here on the "Walking Thing," "Who Pets my Puppy or Dog" and "Five Golden Rules ..."
German Shepherd Dog Forums - View Single Post - New Dog, Very Challenging
Stuff you should know before you get out there with the walk thing. Good luck and thank You for rescuing her.