Need help training my dog!?

Our dog, a german shepard/hound mix, is very protective. He's docile and friendly while we are at home with him, but as soon as someone walks by, knocks on the door or comes inside, he gets very protective. This ranges from close family that he knows well, to complete strangers and delivery men. Usually once the visitor comes inside and he realizes he knows them, he calms down and is very good, but yesterday we had a UPS delivery and he went crazy and I couldn't even open the door. That was kind of the last straw. We have let the behavior go because we kept him behind a gate, away from the door, but now we're looking let him roam the house more and want to correct this problem. However, while I recognize that it's bad for him to want to attack everyone who comes to our house, I don't want to completely stop his protective instincts because we (will soon) have 2 small children and we like that he would scare off any intruders. Ideally, I'd like him to continue to bark at visitors/strangers, but stop on command. Thanks for your help!