furnace blower motor vibration when working hard
I read this post last night when one of the blowers in my system started vibrating, shaking the entire house. The comments above encouraged me to look into it myself, and I wanted to share what I've found in my case for the benefit of anyone in a similar situation.

In my case, it seems the Installation Instructions were left inside the unit and pages of it got jammed on the blades, causing the imbalance.
After I removed these, everything went back to normal. The AC is 6 years old, so these instructions must have been there since the installation, probably taped to a side panel internally.