So it sounds like I have two ways to get this done. Aeroseal, if I can find someone to drive in for a price I can afford. Or brush on mastic, which means I have to pay someone to strip off the insulation along the ductwork, paint 'em shut, and then reapply the insulation. Eesh - this is going to cost me. Are there any other options I need to know about? Worth it in the end, I suppose. Thanks for all the valuable responses.