Pavement=4HI
Smooth Dirt Road=4HI
Rugged Terrain (loose dirt, sand, deep ruts, inclines, declines, rocks, boulders)=4LO in gears 1-3
Severe Terrain (above with major obstacles)=4LO, rear diff. locked gears 1-2
Mud, Snow=depends whether the trail is smooth or has obstacles under this mess. Play with it a bit to look for best traction and control.
Tire Pressure=20 works ok for most rugged terrain.
Others can chime in with more mud experience. It's awful dry here in L.A.
Also, Hummers are made for torque. Watch your RPMs. They should not get too high for max torque over obstacles and rugged terrain. If you hear the engine winding up you're probably in the wrong gear or setting for the terrain you are in meaning 4LO in gear 2 for dirt road at a good clip just ain't right!