The legality is that there isn’t any protected right to shoot trespassers unless you are a Federal cop on a Federal installation, and even then there are rules of engagement.
That’s for crossing your property lines. If someone breaks into your house, that’s nottrespassing, that’s B & E. In most locations if you are inside the house, you have the right to protect yourself by the use of force, up to and including deadly force, against an intruder.
Most locations—but not all. Some localities have ignorant laws requiring you to flee your own home instead of shooting the intruder. This is just about the stupidest law I can think of regarding self-defense, but it is on the books in many areas of the US. The best thing you can do is call your local authorities and be brought up to speed on your area’s laws.
Almost nowhere can you shoot a property boundary trespasser, even if he’s cleaning out your barn with a U-Haul. The reasoning (sound reasoning, I might add) is that if you are inside the house, and someone is outside your house stealing your truck, you are typically in no imminent danger, thus shooting the bastard is assault, not self-defense. Now if he’s shooting at your house, that’s different. In most areas, you have no obligation to flee your home if you are being attacked within it, even if your attacker is outside, and have every right to shoot back. Just be sure to call 911 on your way to fetch your gun.
You can put up the sign; I saw them all my childhood and teenage years, and I’ve even had a farmer unload some rock salt at me for boosting a watermelon from his patch 1/2-mile from his house. People think the signs are cutesie little decorations and get a good laugh from them, but the sign doesn’t alter the legal reality. Shoot a property boundary trespasser, and you’re both going to jail…and the charges against you will be the more serious.
If he’s not threatening you, you let him take the truck and call 911 first and your insurance man next.