I think it's intentionally left up to interpretation to what happened, but I can give a few guesses that could make sense:
1. She might've been strangled or suffocated by the Hitchhiker after the licking.
2. She might've been so frightened by it that she had a sudden heart attack and that's what killed her. You'll notice there's no open wounds on her when the husband finds her in the garage.
3. This one is an interesting theory that I've heard before: that the hitchhiker was all in her head, and he never came back to avenge himself, she kept seeing him in her head because of the guilt and shock of what just happened. She basically had a psychotic breakdown and put herself through the ringer, then committed suicide in the garage (not sure how though?). Although that begs the question, why would the DOVER sign be around her neck if he didn't exist? Kinda makes sense but not really, that's why I would go with the first two as possible explanations.
Very insightful. I especially liked your critique of reason 3. Yes the dover sign shows it all to be real. I hope Stephen King one day will explain what really happened.