If you put 5th Gen rotors on the rear, there's a issue with the rotors centering properly because the hole in the center that goes over the hub of the axle is slightly larger on the 5th Gen rotors and this allow the rotors to be slightly off center, which causes a issue with the parking brake shoes to drag and thusly causes a clicking as the brake shoes flexes and then release.

Some get lucky and the 5th Gen rotor centers up on one or even both if you're very lucky and then the wheel holds it there and you may not get the binding of the shoes but it's just being lucky, for the most part it's best to only use 4th Gen rotors for the rear brakes for this reason, because the hole is of the proper size to perfectly center the rotor.

It's possible that the garage tech is hearing the parking brake clicking and the sound seems to him to be coming from the differential instead, I can't imagine the brakes even of they were locked up causing something to break in the differential.