2013 Express 2500 4.8L extended. Has RPO code GU6 - indicating a 3.42 rear axle. Reviewing the owner's manual, all 2013 Express 2500's came with a 3.42 rear axle. Everything I've researched indicates that the rotors should be 86mm tall, so I order those.

Of course, I pull off the old ones and they are 91mm tall. How do I tell if the previous owner used the wrong parts or I ordered the wrong part?

 

I'll answer my own question.

I test fit both rotors with the caliper brackets. With the 92mm rotors, the bracket was very off, did not have much clearance on the inboard side.

With the 86mm rotor, it was more or less centered.

So the previous owner had installed the wrong parts.

 

That's crazy that they used the wrong rotors. Glad you got it all figured out. You've got a 14-Bolt Semi-Floater. I bet the other rotors are for the 14-bolt full floater used on the Duramax vans.