I can't say I've ever met somebody who is upset that their does AREN'T aggressive. It isn't something you really want to encourage - what if your dog bites someone? What if your dog bites YOU? It's difficult to train a dog to only be aggressive to certain people.
If your dog bites a person, and they complain about it, your dog could get put down - whether or not they were doing something wrong on your property. And even just teaching your dog to growl, or something - teaching them that ANY aggression is good can very quickly lead to more out of control aggression.
I do not particularly approve of what you plan to do - but if you insist on it, please do it carefully. I wouldn't want something to happen to your dogs. (Too, if someone is on your property with a weapon and encounters an aggressive dog, I doubt they'd hesitate to kill it).
Your last sentence... "The dogs should have attacked the stranger." I don't even know what to think of that. You don't WANT your dogs attacking people. People who train attack dogs are always the ones who end up getting their dogs put down, and I quite honestly see it as a sign of irresponsible dog ownership. To train a dog to go after someone on command (like a police dog does) is one thing (that I also don't' like, unless you're an officer) but teaching a dog to go after anyone that it doesn't know is incredibly irresponsible and just fuels bad behavior that will quite easily grow beyond your control. You'd just be proving your neighbors right about pit bulls and increasing the bad stigma about them.
I understand you live in a bad neighborhood, but here's things you can do about that. Self defense classes, pepper spray, heck get your concealed carrying license. Carry a small knife, and always a cell phone. Always lock your house. Just the fact that you HAVE dogs will be enough to deter most people - And I do think if it came down to it, they would step up. My dog is a 200 pound teddy bear whose never even bothered to growl at anyone, but when a dog came after me to attack me he stepped p to the plate and took the hit. (And I could have DEFINITELY pressed charges against that guy's aggressive dog and gotten it taken away.... Just to throw that out there).
Edit: If your'e in such a bad area, why don't you look at moving somewhere else? (Just a thought). I don't think I could handle living in an area as bad as yours seems to be.