Those are definitely 20" wheels. I have the exact same truck except I have the 3.0 and the 20in wheels are standard on the LTZ.Well they somehow got a set of 275/60R20's on these 18s. I think they are similar/identical wheels - just 20" versions.
I'm eventually going to level mine too but will likely go with the 2" lift from GM so I get the longer CVs and I retain all warranty. I'll likely replace the 2" rear block with a 1" to get more leveled.