I absolutely think you should get a trainer. It doesn't really matter whether it's protective or fearful behavior (though my money is on fear, based on your description of him), because it's out of control and getting worse. Having visitors coming to the door is probably a situation they couldn't train for much at a prison program. Dogs don't generalize like humans do anyway--even if they simulated door greetings in the prison program (or he was in a foster home, or whatever), he might see things very differently in your house. Because of this, there isn't really a training technique (as in, something you'd do in general) that will fix this problem. Instead, you're going to have to teach him what you want him to do when people come over.
Think of training more like a process than something that can be done and over with. Every time you handle your dog, you're teaching him something. Hiring a trainer will both help you learn to handle this behavior and make sure you're not accidentally making it worse. Taking your dog to classes may be useful on a few different levels, but I'd also see if you can find a trainer to come to your house and work with you in the environment/situation that is causing the problem.
So, step one is to call a trainer ASAP and make an appointment. I'd actually call several in the area, describe the problem, and ask them how they'd approach it (as a general thing, not like a step-by-step walkthrough). Go with the one you feel most comfortable with.
Until your appointment with the trainer, you need to change some of your management practices for two reasons: one, this is a potential safety issue for your visitors; and two, every time he goes nuts when someone comes over, he's practicing and reinforcing the behavior. It's making your job harder.
If he is crate trained, probably the safest thing to do is put him in his crate when people are coming over. It's not a long-term solution, but it will get you through until you get some one-on-one help.
If that is not an option, then make sure that he is always on a leash when people are coming over. If people tend to drop by just to say hello, ask them to stop doing that for the time being--you need to be prepared for them.
Does your dog have a "watch me" or "focus" command? If not, teach one; if so, work on really reinforcing it in calm situations. Also, do you have other people living in the house with you who can let the visitors in? If so, what I would do is have Axel on a leash either far back from the door or even in another room (you'll eventually move closer, but to start with he needs to be far enough back that you have a chance of keeping his attention). He's on a leash, you have a lot of treats, and you give him basic commands like watch me, sit, etc. Reward him a lot, whenever he looks at you or is calm. If he's going nuts, do what it takes to get his focus back...move around, whatever, and as soon as he looks at you, reward him. Basically, you're teaching him that when people come over, the best thing to do is calmly pay attention to you, not the visitors. If you don't have someone to let the guests in, leave the door unlocked and ask them to let themselves in.
Initially too, you may want to ask people to give you a call when they're at the door, or let themselves in (or have your spouse/roommate/whoever watch for them) rather than knocking or ringing the doorbell. The reason is that your dog may be somewhat conditioned to go nuts at the sound of the bell--like Pavlov's dogs except barking instead of salivating.
Again, just like being able to have Axel at the door, this is something that you will eventually add back in, but your goal here is to keep things under control.
Until you get a trainer's help, I would also try to minimize having people over. I know that can be difficult around the holidays, but it is probably the safest option.
My final piece of advice would be that, since this may very well be fear-based behavior, I would keep things very positive. You want Axel to associate people coming over with good things, not with punishment. He likely wouldn't see the punishment as directly tied to the behavior, but to the presence of the guests, and so it would just confirm in his mind that people coming over is a Bad Thing. Instead, you want to control the situation so that giving you the behavior you want is the easiest thing for him, then reward him for doing so. He'll become conditioned to associate guests with treats and calmness, not with barking and punishment.
If he settles down once people are in the house and it's only them pulling up/entering that is a problem, it wouldn't be bad to have them give him treats and attention, but it probably won't solve your door problem. Guests at the door and guests already in the house can be two very different things in a dog's mind.