There's no visible diff leak, and I won't be able to check until my tools move up here with me.
It may just be the tires - I rotated & balanced them just beforehand, and the fronts are Ultrex, different from the Michelin's on the rear.
Maybe I just didn't notice the whine until I got up here, but I thought I'd check the voluminous hoard of knowledge here just in case.