1989 Cadillac Brougham what is it called

If you are talking of the shim between the differential carrier, and the axle housing, it is a select thickness spacer. Altering the dimension of this spacer will affect differential backlash and preload adjustments, as well as the gear contact pattern. If it is original, it is most likely cast iron, with colored dots on the outer circumference. The replacement spacers have one ring and use a combination of metal shims to achieve the thickness.