The other big difference would probably be the powertrain. Matt, you are right about the Hummer's being gas-powered - at least the H2 and H3 models, but the big 'ol H1 is a 6.6L Duramax turbo diesel rated at 300 hp. This is mated to an Alison 5-speed automatic (read power/fuel sucking) transmission.
Big rigs can be anywhere from 300 to 600 hp diesels, but the gear boxes range from 9 to 18 speed manuals!
Some of these behemoths with proper gearing get much better mileage than you'd expect.
I'm WAY torqued that the smallest diesel you can get in a Ford pickup is a whopping 325 HP, when Ford offers a 200 HP diesel in their Class 4 & 5 delivery trucks. 325 HP is way too much power, the truck is pulling mayber 10% of that at highway speeds, and is not in an efficient operating spot.
The 200 HP motor would operate in a sweet spot in a pickup. But ****upord won't offer it.
Capitalism: The cream rises. Socialism: The scum rises.
This past winter there was a program on "UPN 50 local station" "Cool fuels" This guy had a full size i think Dodge diesel pickup getting 45 mpg on hiway,on bio-diesel fuel. That truck had to weigh 5000lb+. I think it was 4WD?