Thanks for the clarification! So much more complicated than my YJ, which is a simple pull of a lever and I'm in 4-LOW!
Doing the trans filter and topping off all the fluids today. If that goes well, I'll drive it around the block a few times and see how things go from there. I haven't been able to get a CDL light to come on, but it's definitely going into low range without a doubt. Lots of lights aren't working; I wouldn't be surprised if the dash light is out.
Same thing on an 80, pull the lever and you are in 4low with the center diff locked (in OEM configuration). Check out the recent thread with @Proven to go over the CDL light diagnosis.