I adopted my pitbull 5 months ago. He was very withdrawn. Now he is very playful, loving and a fun sweet dog. But only with me. He hides when I am not home. He is terrifed of strangers when I take him on walks. He shakes all over. People want to greet him but he would probably dart into the street if he were not on the leash. I really want him to feel safe and come out of his shell.

A lot of dogs are just naturally timid, it has nothing to do with upbringing, it's just in their nature.
You can try contacting some obedience clubs in your area and see if they have any smaller classes you could attend. With an extremely timid dog I wouldn't like to throw them right into training, but a smaller class with only a few friendly dogs and people would be beneficial.

One of my poodles used to be extremely timid, she was terrified of people, places and other dogs. I started taking her to the local dog park. We would only go in if there were a few other laid back/friendly dogs there, and I would just sit there for an hour or more completely ignoring her.
After a few months of doing this she was actually looking forward to going and playing with the other dogs and started running up to people for a pet.

They key to helping timid dogs is lots of patience, repetition and not pushing them beyond their limits.

Shazamataz is right. Something is terrifying to this dog, and until you can address that, and somehow let him find out that it is OK, he will probably always be a basket case.

This doesn't necessarily mean it was something that happened to him, but may simply be something he is completely unfamiliar with. He has warmed up with you and acts more normal, right? He is satisfied that nothing in the house is out to get him.

Will he ride in the car? If not, start him out easy... let him just sit in the car, with you, in the drive.. not moving or even running. As he gets better, you get out and stand beside it, where he can see you. Don't make this an endurance for him, you have to play it as it goes, and don't move to the next step until he is OK with the current 'lesson'. Next step in this getting used to the car is to start the car and just let it run without moving. Next comes moving it short distances, like around the block, or even just back and forth in the drive. Keep extending the trip as long as he is not upset. Remember, if you go away from home until he gets upset, you have all that distance to get back to 'safety, so plan the trips.

Then, when things are going OK, go someplace where he can 'have fun', so as to make riding in the car associate with pleasures. Some folks think the dog knows when they are going to the vets... maybe because that's the only place they ever take him? Dogs may not be the smartest of animals, but they are not completely oblivious to what is happening.

Once a dog picks up on what you are trying to teach him, learning tends to become easier. He is already trusting you around the house, so maybe he will also be more trusting of your judgement in doing different, more 'scary' things. But again, as Shazamataz said, "patience, repetition, and no pushing past the limits."

Best wishes.

He is probably use to you and he is still little he might be shy of other people,and I would let other people come over let him sniff there hands and give the person you invited a bag of little slices of meat let him or her go up to the dog and give him the treat and say good boy then let him or her go back and show him the treat and say come don't let him off the leash he might be still a little scared and keep reinforcing it then when you think he feels comfortable take him on a walk with a friend that has a dog if you don't have a friend with a dog go to the dog park and find someone with a dog and ask him or her to walk with you