My 2007 LBZ per my Manual for my Dually 4WD Automatic Crew cab long bed is 16K MAX 5th wheel towing and 23.5 GCWR and remember this is with the LBZ Duramax with the new(as of 2006) 6 speed Allison and about 100 lbs more torque than your 2004 LLY, my fully Loaded 2015 40 ft Montana comes in at 15.7K so I am barely squeeking by ,if I had just had a Extended cab 2WD Dually It would be rated at 16.6K but anyway that is why I traded my 2004 Duramax Dually for a 2007 LBZ Dually to tow my 15.7K 5er but like I said my 2004 did it just fine and so would yours I was just a little overloaded and a little short on power on the bigger pulls
