Smitty, did you visit the shrine of the tire-gods?? You must have put a BIG offering in the plate to get away with that load. One good sharp-edged pothole and a tire's sidewall would have been history!!
I'm glad that GM has finally beefed up their truck frames, used to be that that load would have made a Chevy into a 'sway-back'. I see a lot of them around here in Chevy land.
I've put 3000# in my Dodge pickup bed several times, it is a 2500, but the frame is identical to the 3500, just the spring stack and axle tube diameter are increased for the 3500. 3000# and I still had a level truck [properly distributed] and lots of suspension travel, and it rode like a luxury car.