You know, Jericho has a very similar issue. He's never once had a bad human or dog experience (well...since he has been with me. I adopted him when he was 3 months old, so I'm sure he's forgotten anything prior to that), but for some reason is fearful of strangers. I've been watching him and trying to figure out what the trigger is and it seems that it is men with hoods, caps, or people with gloves.
I talked to my trainer about it and she said that it is not uncommon for GSD's to have fear aggression issues. She suggested that when he is in a fearful situation (like ones described above) to get his focus back on me and treat the heck out of him so he begins to associate these situations with treats instead of fear. She also suggested giving the strangers with these triggers the high value treats to reinforce that these are not scary things that they are good things.
It's no easy by any stretch. Jericho is tall, strong, FAST, and scares the tar out of most people when he is in this mode. It's not easy to find a brave sole willing to be your test subject. Especially a stranger. However, this suggestion does seem to be working for Jericho. He's starting to turn to me for treats and my reaction to the "stranger danger". However, if they come too close or try to pass us, he loses focus and it back to the "I'm gonna eat you" behavior. So, while I have not totally proofed this, it does seem to be a solution that is working. You might want to try it with Hunter.