This does not sound like your first post at all. Is he being obnoxious, or is he being aggressive?
Taking treats from strangers and calming right down does NOT sound like he needs to be medicated or that he's a threat. (Other than knocking people down) My bitch NEVER took treats from strangers. She'd by-pass that attempt to try to eat the PERSON. (but only when the mood would strike her, it was a Tuesday, it was sunny, it was cloudy, etc. etc.) Are the people you're seeing familiar with GSD's? Do you have a working line? Is the dog bored? What reactions are leading you to think it's fear or anxiety based? I'm more confused than ever at this point- not that it's hard to get me there.
I really think its a little of both, when he runs and starts barking and jumping on people it "seems" very aggressive, the guy was just walking on the sidewalk when odin got out of the house, his hackles were up, he was growling, when people come to the house he is the same way, but if they feed him a treat he'll take it, and be listen to whatever they tell him to do, but he's always on guard with them and will bark at them if they make any type of sudden movement
I have posted about this problem before, and I tried everything that was recommended by this board, and the behaviorist ( who has worked with several GSDs), but it just doesnt seem to be getting through to him, when he gets into that mode, he doesnt listen, he ignores all commands, He is a great dog with the family, and our close family that visits often, never once will he growl or bark at us, he'll do as he's told, but can be rather cheeky about it,
but the concern most of my family and me have, is that he is getting braver and braver as he gets older ( in a bad way) before he would avoid strangers or things that would scare him, now he meets them head on in a fight or flight situation, but for him it's mostly fight, I have heard the term liability thrown in the post a lot, and the last thing I want my dog to do is cross that line and seriously hurt someone which it seems to me is where he's heading.. I'm just so frustrated, I don't know where I went wrong, and how he got to be like this,