How do I teach my dog that it's NOT okay to growl or bite at us?

I have an 8-year-old standard-sized Yorkshire Terrier (he's about the size of a beagle) and we've had him for about a year and a half. His previous owners found his behavior amusing, and he has improved greatly since we've had him, but we're still having some issues.

99% of the time, he is very well-behaved and easy-going. However, there is no in-between with him: he is either very good and happy, or he turns into Cujo.

We have the most problems when we try to do anything to him, like feel in his fur for ticks or trim his nails or give him a haircut. He just does not like to be messed with, and has even bitten the vet through a muzzle (and he was on "doggie downers" at the time as well), and he has been banned from PetSmart grooming. We are always forced to physically restrain him, and because of this we are not sure he is going to live as long as our previous dogs (just because of the fact that if he's hurt we cannot help him).

His behavior started after he had to spend several months at the vet because he was hit by a car (this was before we got him), and his previous owner has just found out that that vet uses shock collars on the dogs in there so they don't bark- and my dog is very vocal, that's for sure.

At the present, if he starts growling at us, we firmly tell him "no" and remove him from the situation- if he's on the couch, we make him get down by tipping over the cushion. If he's on my mother's lap and is growling at me (jealousy is a big problem when I'm home), I don't leave until he stops growling.

When grooming, he is very food-motivated, and I've gotten to the point where I can trim his bangs if I hold a training treat in front of his nose, and I have taught him that he is rewarded for being patient (by making him sit and stay when I put his food bowl down, and releasing him with a vocal command). The only thing he really enjoys is getting a bath, because he loves getting toweled off and massaged by the water.

I just want to be able to take him places without worrying about his behavior, and how bad he looks because we can't trim his nails or give him that great of a haircut. I'm going to be home for winter break for about a month, and plan to do behavior boot camp with him, including going to the vet for weight checks and putting him on the table, etc, and rewarding him for good behavior. I do not hit my dog- I do not want him to be afraid of me, because that just makes him so much worse as his previous owners used to hit him.