Just bought a 2005 GMC Sierra 1500 and I am trying to make sense of the RPO Codes. Any help would be greatly appreciated. I have a Z82 code that RPOCodes.com lists as Trailer Provisions,Special Equipment, H.O. I also have a Z85 code that is listed as Chassis Package, High Payload, Firm Ride. Are these part of a trailer towing package? I have a transmission cooler but the truck has the GU6 and G80 codes which as I understand it, means it has the 3.42 rear end with limited slip. Would all of this make sense? I would have thought that the rear end on a truck with a towing package would have been the 3.73.