When I removed the part, the main drive gear was still inserted into the blend door. (see photo)
Just as I went to reach for it, the grease let go and it fell down onto the floor of the service compartment. Fortunately I could feel around for it and grab it.
(hearing that rattle around would've been annoying)
The plastics are little chewed, but it just snaps back into its original hole. I think it was designed to do this. The drive gear mount is hallowed. Excessive torque/resistance will collapse the "fingers" into the hallowed space. That's the "gear slip" I was hearing.
Anyway, O'Reilly Auto has a lifetime warranty on the Dorman part. I intended to exchange but was presented with the option for refund, so took that instead. If the original OEM passeneger ever fails, will be picking up another Dorman.
I temporarily plugged the original OEM into the wire harness and turned on recirculate to get it aligned with the little arrow (see photo of Dorman part) on the case. Rather than unplug, turn off the ignition as soon as it aligns. Original OEM reinstalled and working flawlessly - no noise.