very fearful dog with barking problem

Question
I have a very very scared dog.  He acts fine around our family but when anyone comes over, he runs around barking constantly.  When we first got him, he would hide when he heard the doorbell.  Now, he just runs back and forth barking like crazy.  If anyone tries to approach him or even talk to him, he urinates. I don't know what to do with him.  I've tried ignoring him, tossing him treats, and even putting him in another room, but nothing is working.  We bring him out often, to baseball games, parks, daily walks.  He still is very afraid of people (crouches down low with his tail between his legs and trying to run away).  He has been through obedience school and his teacher called him an "extreme case".  I don't know what to do with him.  I'm at the point now where I'm avoiding having people to my house because he's such a problem.  Please help, I'll try anything.

Answer
Nicole, what happens when you put him in another room or in his crate? Does he continue the barking? Do you encourage people to approach him and try to pet him? That could be contributing to the problem.

I would strongly suggest that you have a local behaviorist/trainer come to your house to work one-on-one with you and your dog on this.