Wow... I'm sorry you're going through this with Jack, Ashley. I know how much it sucks. =( *hug*

Honestly, the most important part of dealing with any sort of aggressive behavior is management. You need to set him up for success, by never allowing him off-leash in the front yard. Never, ever, ever. It is much better to keep him crated while you're gone than for him to have any opportunity to get out.

Spend time walking him around and have meet & greets with neighbors. They should kneel down, allow him to approach them when he feels comfortable, offer him treats, and pet him when it's appropriate.

Gonzo is similar, in that he is very, very wary of strangers walking near the house. The issues stopped completely after I made it clear to my family that I will $#^&*^& them up, if they ever allow him out in the front yard. Even though my neighbors probably thought I was a weirdo, I introduced him to all of them, and when ever we're on walks I ask them to say hi to him.

I can't say whether or not I agree with this behavior being the product of an unstable environment, or dominance. It's obviously a protective behavior. It has probably been reinforced, unintentionally, many times... Jack sees a person near the house, who is on a walk, Jack barks/chases, the person continues walking. Jack thinks to himself, "Hey, I showed them!", and the behavior is reinforced. I agree that NILIF could help in general, but I don't know if it would solve this issue. Preventing any & all chances of him being reinforced, by never allowing him off-leash in the front, is my advice. Good luck!