Sam,
You are correct, as my truck was built with 3.73's, I am over the GCW...however I have changed the ring and pinion to 4.56's, added air bags as well, and have an exhaust brake for slowing it down. Those weights were without the two motorcycles, which will add about another 1,500 pounds to put me somewhere around 27,500 (I plan on scaling again this weekend). My truck tows fine and more importantly stops fine, and I am 100% confident going up and down the road (former over the road driver). The two big differences between our trucks is you have the 6.7, I have the 5.9, you have the automatic, I have the 6 speed manual (NV5600). The only other issue, is the legality should there be an accident. I am aware of any ramifications of MY actions, nothing is unsafe, axles, tires, suspension, and frame are all rated for the load they are being subjected to. The number is the number, and as manufactured it is 21,000, as it sits, your guess is as good as mine, but In my very eperienced opinion, my set up is good. YMMV
Big Mike