I'm not sure what the military calls it either.
But it has frame mounted zf planetary axles with gear reduction and internal brakes (axles are mounted like in some tractors). It has full hydraulic steering, the 6.2 diesel, TH400, and an NP208 transfer case permanently in 4WD (I've read some say it's locked in 4-Hi some say 4-Lo). And I think they have a 22:1 axle ratio.
They reportedly weigh 12,000 lbs and have a 60,000 lb tow capacity. The front bumper and the box behind the cab are ballast weight.
So I would expect 20mph to be totally maxed out at 3600rpm. Take the huge front bumper and the rest of the ballast off and you might hit 21mph.