Hi guys, thanks for your responses.

The calipers on the 4.5 rear axle (and car) are rusty, but the JY also has a 6.3 rear axle that has some "silver" calipers. Not sure if these are new, aluminum, or silver paint, but they look fantastic. Both rears have solid non-vented rotors.

The 4.5 is in a car, VIN available, but the 6.3 is in a pile of axles.