Let me ask you this question. If I become the target of someone's road rage and they follow me through a parking lot. When I stop in the parking lot he comes up to my car, yelling and screaming, and starts pulling on the door handle. Am I allowed to threaten/use deadly force at that point?
As usual, PM's answer is spot-on. Your job is certainly not to confront the road-rager, but to call 911 and meet up with a cop ASAP. Drive to a police station if you have to.
Yelling and screaming may signal an intent to do you harm, but if you are locked in your car the road-rager really doesn't have the Ability to do you real harm. You would be hard pressed to explain shooting someone in that position.
Even if they start pounding on the car with their fists, they still don't really have the Ability to harm you. At worst they would cause some property damage to the vehicle, and as pointed out above, you are not allowed (in GA) to use lethal force simply to protect property.
Unless they escalate to using a potentially lethal weapon themselves, you really won't have a good legal defense for shooting someone who was simply yelling at you. The old 'sticks and stones' rhyme sums up this perfectly.