I've had the same problem and what I've found is the rebuilt alternator is the bad guy. On the side of the alternator with 2 legs, one is bolted down solid and the rear (towards the drivers side) has a sliding bushing that is supposed to slip and make contact with the support bracket. Problem is that when this bolt is tightened, the bushing doesn't slip and a bending force is applied to the leg. This contorts the rear frame of the alternator and causes the rear bearing to lock up or drag heavily. This adds to the drag of the alternator and causes belt slip even if the belt is tight. This also causes a noticable whine from the alternator even when the belt isn't slipping and squealing.
Quick fix:
Loosen the lower bolt on the alternator that's toward the drivers side (away from the belt). If all gets better instantly, that's your problem. I used an Autozone rebuild and 2 in a row did the same thing. Get a warranty replacement and make sure that the slip bushing in fact will slip when the bolt/nut is tightened. (a hammer helps) (a knowledgeable rebuilder is better) I intend to be a pest until they get it right.