Things worked out well and that is just fine and dandy and is what counts, but things could have worked out better.
As noted, you should have called the cops when you saw the drunk driver.
Given you assessed the guy as a drunk, after he slammed on his brakes (hard) the first time, you should have backed off.
As you didn't back off and the guy repeated the procedure 3 more times and you decided that he was pissed at you, you should not have stopped anywhere near the guy when you came to the light. Stopping any closer than a hundred feet or so was not a very good idea.
Contrary to your post title, you DID draw your gun...and simply staged it to the seat. In other words, you drew your gun in anticipation of a potential conflict.
And contrary to Steve13's post, if the guys in the van approached your vehicle with non-gun weapons, there was zero reason to run them over with your car given the situation. When the guys got out, you simply needed to reverse away from the situation as you did not have a car immediatily behind you. I don't doubt your vehicular assualt actions would likely be justified in such a scenario, but since you were protectively enclosed in your vehicle, that said weapons are only good at contact distances, and the fact that the guys could not run as fast at you as you could reverse, why deal with the legal hassles that would come with running them down?
