There is another very effective method for taking auto fourbies down steep hills (slippery or rock hopping), often referred to as "driving through the brakes". My Pathy had a lovely auto box that had very good engine braking in low first but it would still tend to go too fast especially if you were crawling over some rocks down hill.
Here's the gist of it, see if you can get the hang of it after a bit of practice.
Basically you select low first (or reverse) and apply modest, steady pressure to the brake with your LEFT foot. Enough to hold the car with the handbrake off, then gently apply power and the car will move slowly in a very controlled way, in fact dead slow if you want. If you lift off the gas and it gently stops then you have about the right brake pressure. On muddy slopes you will need to keep on enough power to keep all the wheels turning slowly and steadily whilst keeping absolutely constant pressure on the brakes.
A few of things happen when you use this technique,
1. you tend to fool the open diffs into seeing very similar torque across the axle so you get a very even drive across axles, particularly true on the front axle where your brakes become the dominant load on the axles, not the tyre grip (or lack of it)
2. you get all four wheels receiving very close to the same drive torque and nice and slow. You can suddenly feel the car biting on all four wheels insead of slipping and sliding as it will do if you attempt to roll down the hill on brakes alone.
3. your torque converter is slipping heaps (you are forcing it to) but your engine rpm will be somewhere around 1000-1500 (not at idle) and that usually means the torque converter will be trying hard to connect engine & tranny .. this is one you would have to ask an expert about, but I have heard claims that this is one of the reasons this technique works so well.
I have crawled down slopes and rock ledges where I have got on three wheels momentarily but despite a little bit of slip the car quickly regains composure as you still have all four wheels (restrained by the brakes) being driven nice and steadily. The same piece of track tackled on brakes alone would have been pretty dicey.
I have found this technique very valuable whenever you want to crawl an auto fourbie through a hard bit and you can easily go as slow as a Tojo diesel in Low first. In fact using this technique I could often get through better than manual diesels with open diffs
My guess is that you ATF fluid heats up a bit doing this but you are only doing it a couple of minutes here and there, plus your front discs may makes some weird squealing and rattling noises as the calipers move about under the combined forces of braking and drive. Nothing too alarming though as they are still working
Some 4WD training instructors teach this technique.
Go out and practice it and soon you will be using it often in the hard stuff. When you are good at it, you just apply a bit of brakes with the left foot whilst maintaining or slightly increasing the power anytime you look like losing momentum UPHILL because it helps even out the cross axle torque (poor man's LSD).
BTW, don't forget the key factor of tyre pressures. Let them down to 25psi and that alone will make a big difference.