Anybody know about 2 speed rears?

[k72516]

I'm looking at this awesome C-60... or C-65, I don't know what the models exactly were for the medium duty trucks... It's just one of those huge big square dually box trucks. 5 speed, with a 2 speed rear. But I never really got what the 2 speed rear was all about. All I know is that it's for when you have a really heavy load, you do something and it gears it down real low so you can get the load moving, at least I think that's what it's for.

Short version: What does a 2 speed rear do? And how does it work... do you just flip a switch to go between high and low? Do you think it would fetch any money if I wanted to swap it out, or is it just a worthless relic?

Also, is it true that they have four drum brakes or something? I read that on the only site I could find about 2 speed rears, but it was geared toward older pre 60's trucks, this ones a 70 or early 80 something.