How can I get my Lab to stop being skittish?

A group of kids down the street used to come up and down kicking the fence and taunting my Lab for fun and now my dog is very skittish. The kids have moved, thank god, but my dog isn't the same. She used to be a very loving playfull dog but now if its not me or my kid coming in the yard she barks out of what seems to be fear but if they walk toward her she runs to the corner and cowers down. I would like her to go back to normal where she would bark at people she didn't know but stay calm or even get excited when a friend comes over. Before those kids she was very protective over my kid and has bit the UPS guy when he walked toward my son. Since, she won't even go near a stranger even when they walk in the yard or approach my kid. Is there anyway to get her back to normal or at least control her nervous outbreaks?