I have a Powerglide that shifts into second gear at about 21-22 mph. I'd like to get it to shift at 15 mph or so. Checked the modulator and vacuum line, and everything is tight. Is the stock modulator adjustable, or just the aftermarket ones?

Just the aftermarket ones.

Okay, thanks.

The shifting event is controlled by the "amount" of open throttle (read vacuum) one gives, ie, the lighter the throttle, the higher the vacuum, the lower mph the upshift occurs.

Are you sure you're giving the car the right amount of throttle? You might manually control the shift points by using a MityVac and control the amount of vacuum to the modulator. Less vacuum, higher shift point, higher vacuum, lower mph shift point.

All is true if the oil pump is not worn and making the designed oil pressure. There is a tap to place a meter, 0-200 PSI, I think, on the drvr side of the main case. Investigation on this would be the rule.