use autozone part # 55054 (front rotor)

They are probably asking because the Silverado 1500 came in regular duty suspension (6 lug wheels) or with an HD suspension (8 lug wheels) the latter trucks are all crew cab short box 4x4 trucks that I've seen.

You should be able to find your brake code on a sticker located inside the glove compartment. It's been ages since I had to decipher brake codes out of a jumble of digits, so I can't explain to you the code's exact location on the sticker. However, look for one of those two codes, and when you find either one, you'll know that's the right one. Should you not be able to locate the sticker, simply look up the rotors for each code. Most parts stores have a "picture book" which contains specs for each part number a given manufacturer offers. Simply copy down the specs, and then measure/compare to the parts on your vehicle. Often times, brake systems are decipherable by easy-to-get-to parts like power boosters and master cylinders. (Obviously easier than measuring brake rotors.) Most hydraulic brake parts catalogs contain a listing of GM brand brake codes and the specific components each system has. It's easy to rule one brake system from another if, for instance, one system has a dual diaphram power booster and the other has a single diaphram booster. All the above said, it seems to me that you need to find a better parts store with knowledgeable counter people. Honestly, a good counter person should have figured this out for you (or explained how to figure it out) in a matter of minutes. Unfortunately, they are hard to come by these days.