Z71 RPO started in 88 (as said above).

The package has changed slightly over the years. Usually includes different (stiffer) valved shocks. Front differential skid plate, transfercase skid plate, sometimes a fuel tank skid plate. That usually it. The spring rates can be the same for a Z71 vs a non Z71. The rear locker G80 RPO is not included in the Z71 package, it is an option with or without the Z71.

My '97 is a Z71 has a crappy aluminum front diff and xfer-case skid plates. Bilstien shocks (not sure if they were factory) and a plastic cover on the fuel tank. Oper diff rearend. My friend had a '92 Z71 and had the same stuff as me. He now has an '01, just incudes skid plates...