Jim came out the first day that we were available to fix our vibrrrrating blower. We were slightly worried that they were going to have to replace the entire blower, but it turns out that Jim just replaced the motor.
The motor in the Space Pak blower is a real mover, so to prevent it from being a shaker, it has several weights attached to it to add stability. It turns out that one of the weights on our motor was missing entirely, and so the motor was woefully unbalanced the vibration. Kind of like when your washing machine starts shaking when all your clothes get stuck on one side--except in this case, the washing machine is is spinning at several thousand revolutions per minute.
It only took Jim about 1:15 to finish up the work, and now we're enjoying our cool and quiet house.
Oh, and zoned cooling be damned--I'm happy to say that the temperature difference from the first floor to the second floor is less than one degree.