The DuraMax diesel is a good option. I'm actually surprised you found one in stock, they are generally very rare, almost a custom order item.
GCWR is 17,000 pounds so you still cannot quite tow 10,000 pounds if you have any decent load in the van. Also it's detuned from the truck version, at 250 HP, so it'll actually climb slower than the 6.0 gasser, at least at lower elevations. But it will get going easier, and be more civilized since it'll stay at low RPMs most of the time (it does make lots of torque!).
Also before the front axle is the same as the gasser vans, rated at 4600 pounds so there's no extra capacity for an aftermarket bumper. It uses the GM 6-speed not the Allison, so I don't recommend pushing the GCWR because the torque converter is at its limits in that application. I was considering one myself since my V10 has 422,000 miles already, but the limited front axle capacity and torque converter problems made it a no-go for me, since my combined weight is usually 22k and can hit 25k on some trips.