Look - it doesn't matter how damn slow you have to go if it keeps you off the brakes. I'm not at all familiar with the Jeep autoboxes - if it isn't possible to lock the transmission into a gear, that's an inadequacy of the transmission and I'd be looking for something different. But if that's what you have to deal with, then the only way I see out of it is 4-lo. Too slow? Too bad! Beats brake fade.
Anecdote: I was going down the eastern slope of Tioga pass in the High Sierra, pulling a fifth wheel trailer. The auto-box wouldn't hold it back enough in 2nd, so I put it in first. Pulled over at EVERY turnout (there are plenty of them), and got honked at by all the flatlanders as they flew past me, feet on the brakes until they vanished out of sight ahead. WITHOUT EXCEPTION, I'd soon be smelling burning brakes, and there the fools would be in a turnout a couple of miles ahead, with their heads under their vehicles wondering why their brakes were fading. One case looked like the guy had barely managed to get it stopped before going over the cliff. I'd just toot my horn and roll on past. I don't think I ever saw any of them again - I figure they got religion after that.
Gear the fucking truck down as much as necessary to stay off the brakes. When you do need to brake, do so like you mean it. Slow waaaay down, then get off the brakes again to let them cool until the next time. Otherwise, sooner or later sayonara baby because physics wins - EVERY time! Again: Brakes are designed to slow you down, they are NOT a speed maintenance device. That's what the engine and driveline are for.