When did chevrolet and gmc start using the z-71 package?What is the difference

in that and normal 4x4?? does the z-71 have heavier duty parts??I am asking this question because i have a white 1994 gmc ext cab 4x4 and my freind has a 1994 white ext cab z-71 and they are the exact same truck!! except for the grill and his says z-71 and mine says 4x4.I am pretty sure chevrolet started z-71 in 1989 but i am beginning to think gmc didn't start using it until 1995 or 1996 because i have never seen a pre 1995 gmc using the z-71 badge.
also i forgot to add to my freind's truck discription that it is a chevrolet not gmc like mine.sorry for any confusion....
ok guys you are only answering part of my question one thing i really wanted to know is if gmc started using the z-71 badge several years after chevrolet.

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...