Well, mystery solved. Got a call from the service writer today.
The "caliper guide pins" were completely corroded and "frozen" on the passenger rear brake caliper.
The opposite side was "sticky" and on it's way to being like the passenger side.
So, new calipers, rotors and pads for both sides. I am relieved. Have to believe that driving in wet, road salt covered roads every other day this past horrible winter may have had a hand in this.
Should be done Monday afternoon. Thanks for all the replies and suggestions!