My wife and I like to take my dog Shadow (105lb male GSD 3yrs old) on hikes.
The other day at the lake we were hiking the trails (on lesh) and, a couple was comming down the trail from the other direction. I stepped about 3 feet off the trail and sit my dog so they could walk by us. Right before the man past us he turned and quickly held his hand in front of my dogs face. I had Shadow on a flexy lesh in all the way and locked, but Shadow graweled and barked and almost bit the man. I corrected Shadow for this. I was not really sure after I did it that correcting him was the right thing to do.
Should I or should I not correct him?

I don't think it was a good idea to correct the dog. His aggression was reinforced because the man ran away so in your dog's mind, he was successful at driving the jogger away. You might want to have some volunteers run by him and throw a handful of treats and keep running away. Do this over and over with several different people many times(starting with people he's familiar with and then to the unfamiliar). It's better that he think that people running by him are good, than to brace himself for the correction he's going to get because that strange person is running by.

Don't get me wrong. I definitely believe there's a place for inhibition. You SHOULD get some well proofed basic obedience on your dog. Your dog should have a rock solid recall. He should know that you make the decisions, not him. Don't misinterpret what I'm saying and think I'm suggestion to beat or mistreat the dog. Not at all. You should make sure your dog's obedience is well proofed with CONTROLLED distractions before you put him in situations where there are uncontrollable distractions.

Seek professional help in your area.

I take one thing back. For that one instance, I believe you were correct in correcting him for that only because you had no other way giving him a consequence for his behavior. it would HAVE to be inhibiting though. In the long term it's not good for the dog's attitude towards strangers at all. And if you don't inhibit his aggression you will end up really stimulating it and reinforcing it.

oh.. Ok.. I like the idea of having people he knows come up with treats and then make it people he does not know. I have no problems with the recall, down, sit. I have only had one instance when a rabit ran buy and he took off that he did not come back and I had to go and get him. I used Ed's ob tapes which are great!
While I am doing this training with people walking buy and giving him treats what if he barks and grawls at them. Do I correct him? I use a prong collar. He only needs around a level 2-3 correction out of 10.

You restrain the dog. People are jogging by (friends first then strangers, not all in one session either. First couple of sessions, use friends, next session use a friend for the first repetition and then use strangers) TOSS the treats from a distance (a few feet, where he notices a person but does not necessarily act aggressive and then close in as he progresses) and repeat this sequence over and over again with different people in different places. You should correct if he's pulling you on the leash, not if he's behaving aggressively. The food will teach him that people are good. And of course, use a food he likes!! Not low level kibble. Some hot dogs, or cut up natural balance rolls.

Using a level 2-3 (and of course this is sooo relative according to person and dog) correction will only heighten the aggression.

Great! Then I will set it up and give it a try this weekend.

Thanks for the advice. I will let you know in a few weeks how it is going.

I like to use Liver... He goes crazy for liver. I am not sure why it stinks like dead fish.