Rear axle noise starts at 20 mph and gets worse?

If the tires are good, I would support the vehicle on a lift and engage the rear axle. With a mechanics stethescope, you could move the tube to different locations around the axle to identify where the nose might be coming from. Rear end nosies are usually due to outer bearings, carrier bearings, or a ring & pinion that has frosted or pitted.

Many times I will replace the fluid first, and see if that eliminates any nosies due to lack of friction reduction in the gear oil. I always recommend synthetic gear oil.

I would stop driving it because it sounds like the U joints are out and the driveshaft could come loose and cause damage. Use a flashlight to inspect the U joint and look for rust colored dust.

This guide can help:

https://www.2carpros.com/articles/comprehensive-guide-to-checking-and-replacing-an-automotive-drive-shaft-u-joint

Here is how to remove the driveshaft in case you need to remove it to replace the U joints.

You are welcome, please let me know you find.

It sounds like the transfer case is having a problem or the U joint is locked up which it could still look good. I would remove the shaft and manually move the U joint to check the operation. If it is okay, then for sure the transmission case will need to come apart for inspection. Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.

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Yes, you can.

Yes. That is a selectable 4X4 not like AWD where it has to have both axles driven and disconnecting one end would stop the vehicle. Your truck you can shift it in or out. When you are in 2W high you are basically driving a 2-wheel drive but carrying the extra weight of the front axle and transfer case. Remove the front shaft and keep it in 2WD.
You actually have the luxury of being able to remove either driveshaft and still drive on the other end! I drove a Blazer for a year using only the front axle while I built a new rear axle for it. Strange thing is that it got better mpg running on the front axle than on the rear!

Yes, but these days nobody wants to get out to lock them in. Some you can but it depends on the axle design. On the full independent ones you can't have true locking hubs, on the older link and coil design with the solid axle you can.