Backing up a steep driveway in 4WD Low gear?

YES........

Use 4WD low & slow

One of the practical reasons for 4WD

I do it all the time. Works great!

On a hard surface? I would not use 4WD. Gravel or dirt driveway? You bet.

Sometimes you gotta do what you gotta do. I think the wear and tear will be less on the 4 wheel drive components in 4low than on the transmission in standard 2high/4high.

It depends. When I had my F350 I had put WARN hubs into the front end. That meant that I could use 4 X 4 low range and still only feed the rear wheels, or I could put the hubs in and have 4 X 4 front and back.

It certainly was a handy feature when I needed to go slow and steady. So, certainly on pavement and making sharp turns isn't good, but it does depend on what happens in your differentials and what gets driven.

Isn't "that" what 4 low -- forward OR reverse is for?

If it causes you worry, simply spray water on the surface. That will help the drivetrain a little and prevent tire marks on the concrete.

If your 90 deg turns are on the flat then disengage 4wh when you make the turns. Turns when in 4wh are really tough on the truck. A sharp turn with 4wh engaged will feel like you have the brakes on even if you have no load.

Yes, it helps the engine and slow down backing.

It also seems to have a more controlled backing maneuvers.

And yes, I'm new to backing trailers and, maybe psychologically, it helped somehow.

The only downside is that my 4runner's beeping is annoying when the 4WD is engaged.

It really would depend on how often you will be doing this. Once or twice shouldn't hurt anything. But if this is something you will be doing often then it is really going to cause a lot of wear on the driveline, or worse. If it were me I would stay in 2 wheel drive high for both 90 degree turns and do the straight part of the hill in 4 wheel drive low. The "binding" that occurs when turning while in 4 wheel drive needs to be able to release the pressure, which is done by slipping the inside rear tire and the outside front tire. It will be even more difficult for that release of pressure to occur with the additional 7000 pounds of weight on concrete.