[DWPC]

I'm guessing, but I think your expecting the clutches in the trans to do something they weren't designed to do; upshift under "negative" torque from engine braking. OTOH, if you weren't supposed to do that, the ECU should have been programmed to prevent shifting in that condition. Or maybe the Hummer folk just didn't anticipate it. I never upshift going downhill in low range. If it starts sliding, I'd apply power. At least thats how I'd have done it in my old Montero; I haven't had the H3 in that situation yet.

Too high a gear when going downhill may leave you with no engine braking and too much RPM to downshift while keeping traction. I'd be interested in knowing if any 4x4 w/auto can do what you want to do.