Personally I'd look at issues with the B+ cable or grounding before I threw *any* money at a GM alternator. Of all the versions I've only been failed by shaft bearings...never a failure to charge.
Easiest IMHO is to hook up a voltmeter and see what the system is charging at. Worst case for alternator is when the car is idling and you turn on every electrical accessory known (heaters, rear window defog, max blower, etc). As long as the battery doesn't go into discharge at that point, life is good.
My headlights on my 00 GTP are dimmer but that's because the headlight lenses aren't transparent any more. Rockauto has the parts but I have other spending priorities....or.....rather.....lack of spending priorities
Also...saw your location as AZ....if you get 4 years out of a battery you deserve a blue ribbon or something.