Well... One thing, the main reason hummers (mil) can not be used on civilian roads is that their running gear will not pass a DOT inspection. The axles are bulletproof, and use the Portal system developed by mercedes. The Civ version also uses these axles, but they had them DOT approved. Having been unable to take the two apart beside each other, I have no idea if the two are the same or not.
Gm may just have had the government issue equipment DOT apporved.
The hummer (mil) also hasn't gone through crash testing... So they won't let you use them.
Ok, for the record:
Mil vs Civ
Runflat Tires / Kevlar Reinforced Same
Central Airing / Deflation system Same
Bulletproof glass Option
Portal Axles Same
Complete compartment sealing Same (witnessed)
3 speed Hydramatic Various, 5 Speeds avail.
Of course, The civilian issue hummers are the only ones on the road. The US government upon decomissioning the humvees, cuts the hummers into halves, and transfers them to scrap yards.
If a civilian is caught with a military spec hummer, then they are fined (DOT violation) and the hummer is cut in half again, and returned to the junk yard.