This is already a reality. It's called Ranch Rescue. http://www.ranchrescue.com/
Their website is being upgraded right now, so there's not much to look at. But it's basically a citizen militia that wants to protect property owners near the border.
Since its inception, it has been harrassed by the government at all levels. It has become VERY clear that our government does not want to shut down the border problem. They want it. They need it. They would rather shoot Americans who defend our borders, than the illegals crossing over it.