Sounds to me like its pulling to the left rather than drifting to the left. As you suggest above, even when the road cambers off to the right, the car goes straight ahead and not towards the right hand side of the road, which suggests it is fighting the camber. If you have had allignment done on a hunter system, it would point to something being either worn or the allignment wasnt done right.
Did they do a full geometry check or just the basics? As I found on my car that it pulled slightly to the left (not as bad as yours), and it was the rear wheels that werent straight, they both pointed off to the left. If your allignment guys thought the rear was straight and true (as can quite often happen), and they alligned the fronts off that, then the tracking will be straight, but the car wont be, if that makes sense? They base the "trueness" or straightness off the rear wheels and assume they are dead straight, if they are not, it can cause your problems I think.
If all was done right in the allignement, then some suspension or steering parts may be worn. In that case, its best to wait for someone like ZMANALEX or Abbey to comment, as they will know more.