Huh, this whole time I had no idea that the SS had dual piston rears. Looks like according to Rockauto they are just front calipers in the rear!
FitsRear Left; Silverado 1500 HD; with Phenolic Piston Caliper
OrFront Right; Silverado 1500 Base Model; Rear Disc brakes; with Phenolic Piston Caliper; with Brake Code JC4; GM Brake Codes are on the "Service Parts Identification" sticker.
OrFront Right; Silverado 1500 Base Model; Rear Disc brakes; with Phenolic Piston Caliper; with Brake Code JH1
OrFront Right; Silverado 1500 Base Model; Rear Disc brakes; with Phenolic Piston Caliper; with Brake Code JH2; GM Brake Codes are on the "Service Parts Identification" sticker. (Only 6 Remaining)