I really hope the OP hasn't stopped reading, because there IS some good advice in this thread.
I experienced a similar thing (albeit on a much smaller scale) with my GSD resuce, Heinz. Took the sad, malnourished dog home, rehabbed him, and for about 4 months everything was great. I thought I had lucked into the world's nicest dog - nothing upset him, I could take him anywhere! He was perfect!
And then the issues started. My formerly happy-go-lucky, sweet loving dog who would let ANYONE pet him was turning into a barking, threatening monster whenever a new person approached! I thought I had rescued an ill-tempered, aggressive beast who was just now showing his true nature. I was afraid to let him near people.
Thanks to some threads I found on this forum, and the advice of other experienced dog owners, I sought help at a local boarding kennel with a good training program. Heinz and I had a consult with one of their trainers, who thought that some of his aggressive behavior was due to his overexcitement and my lack of clear instruction. We enrolled in a group obedience class and I learned a lot about working with my dog - mostly how proper timing of the release/reward is everything!
Anyway, I learned how to make him more comfortable in strange situations by being a good, consistent pack leader (note I didn't say "Alpha") and giving him clear instructions. It helped us immensely. And I learned that if I have to pack a pocket full of treats to distract him when we go into a high stress environment, it's not a moral failing on my part
(I have a very treat-driven dog)
My opinion is that GSDs like to Do Stuff. If you don't provide them with Stuff to do (in the form of a command, or a job, or some kind of redirection), they'll come up with something on their own. Which might not be what you had in mind