#1, hauling 5-1/2 tons behind a p/u rate for 9200 by itslef isn't anything to sneeze at. I see folks overloading half-tons rated for far less hauling camper weighing jsut as much doing it all the time, but I give them a wide berth.
#2, I wouldn't be overly concerned about being a mere thousand over your GCWR. Though a lawyer after an accident would have a field day with you and your insurer, I don't think it would hurt the truck any.
The only numbers on your door sticker are for individual axle ratings and the GVWR (truck only, and you've already noted that GVWR will be less than the sum of the axle ratings). Nowhere there will you find the GCWR (a rating for the total of your truck and what it's pulling). The GCWR is more typically found in the owners manual or a separate publication. I'll ignite an argument by saying that the manufactureres aren't required to publish those numbers, but do so in order for customers to buy a vehicle suited to their needs. Still, a lawyer will go to town on them.
The factors in GCWR are many, IBNLT engine hp and torque, transmission, rear-end ratio, suspension and frame strength, and BRAKES!
Do a little googlin' and you can probably find the tables that will show your diesel will handle more weight than the gasser.
That said, what you don't want to get caught exceeding are the numbers on the door sticker -- the GVWR of the truck, weight ratings for either axle, the load ratings of the tires on those axles, or your hitch's rating. If you can stay within those ratings by setting up your loads so that the hitch, axles and tires on the trailer aren't overloaded either, you should be good as far as enforcement goes, provided that the balance of the load handles well.
But I capitalized BRAKES above for a reason. You can have the most gawdamighty, chipped-out, smoke-belchin', turbo-screechin' power plant imaginable with armor-and-gold plated driveline components, and a suspension to carry 8 tons. That's wonderful if that's what floats your boat. But if you did all that work so that you could get a heavy load up to speed with acceleration similar to a a peppy coupe, all the upgrades are useless if you can't stop it.
I've got a long-bed-long-cab Ram with a Cummins and single rear, big disc brakes all around. It's rated for 9900# GVWR and, with the weight of the B&W hitch frame, a full tank of fuel and my bloated carcass in the seat, it scales at a whisker under 8k. Dodge says my curb weight is 7220# and GCWR is 21,000, leaving a max trailer weight of 13,650. (They round a lot in a couple places, so the arithmetic ain't quite exact there). So if I adhere to their limits, I'm down to 12,850 in round numbers on the trailer.
With that in mind, I had my gooseneck built with 7k tandem single axles. Two thoughts in doing that. It keeps me from getting big-headed about what I can haul, and it attracts a lot less LEO attention, especially behind a p/u with single rear, even with two old tractors chained down on it.
Trailer itself scales a whisker over 5k, so I've still got almost 8k I can put on and stay within specs. My tractors are smaller Farmalls (BN and SuperC) so the only time I've pushed that limit was hauling four ton of wood pellets on pallets for my step-daughter. I had them load it with two ton centered over the trailer axles, and the other two ton centered between them and the front of the deck. Just for giggles, I scaled it on the way to her place, and everything was fine.
Still I think brakes are one of the biggest limitations on GCWR. I've got a six-speed hand-shaker and an exhaust brake on my p/u. With a maxed-out load like that (or even with the tractors on board), I'm usually wound up tight in one gear lower than I'd use breathing easy running bareback, so the exhaust brake is doing all it can. Using all the prudent driving practices that one should WRT safe following distances . . . it was an easy drive.
The only thing I'd add is to encourage you not to get hung up on the visuals, i.e., what looks like wasted deck space once you've maxed out your weight. Y'ever see a Class 8 goin' down the road with a 48' flatbed and nothing more than two coils of steel that only take up 16 feet of the available deck space? There's a reason for that.
For parting wors, I'd just say that diesel pickups when compared to the gassers of the same make and year are wonderful things. They'll out-pull their brothers. Just don't lose sight of the fact that they have their limitations.
