Before I explain what lead me to this 4x4 problem, I'll simply say what I'm currently dealing with:
I have no front-wheel drive on my 2002, 2-door, 4x4 Blazer. The transfer case shifts. The 4x4 buttons engage all 3 modes: 2 HI, 4 HI, 4 LO. The clutch gear in the front differential doesn't seem to be engaging.

Note that I haven't done any work yet to fix anything. I'm still mentally troubleshooting.

I'm planning to check that vacuum actuator under the battery to make sure it's not corroded and for any loose vacuum lines. I don't think I have a vacuum problem, though. If that ball-switch sensor in the shift cable for that acuator is doing what I think it's doing, then it's not a vacuum problem because I get a light for each 4x4 mode I switch to.

If that's not the problem, then I'm going to check that clutch gear itself by removing the shift cable and manually engaging it to see if I get wheel spin from rotating the front drive shaft.

If I get no wheel spin from that, could it be that the gearing on that part is totally worn out?

I'm checking to see how much of a job I'm looking at here if the problem goes deeper that what I meantioned. Do I just need to replace a few parts, or am I looking at a full differential rebuild here?
Has anyone dealt with this particular 4x4 problem before?

Videos that lead me to this conclusion:

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So, what lead me to uncovering this 4x4 problem?
I was triyng to troubleshoot another grinding sound problem I've been having for several months now.

This post here was when the noise first starting appearing. It was very intermittent and only noticable once everything got to operating temperature.
https://blazerforum.com/forum/2nd-ge...-right-102683/

The noise started appearing after I had changed out transfer case fulid and lube in both differentials. I am very certain I refilled them with the correct amount then.
I have also replaced both CV axles because the driver side axle had a torn boot I've been driving on for quite a while now and thought that was the source of the noise.
After replacement and a test drive, the same noise and grinding feeling under my feet still persist. And it's only while turning right.

Being unable to troubleshoot this further from inside the cab, I got a friend to help me jack all 4 wheels off the ground, put it in drive with 4 HI, and make a right turn while I listened from outside.
Doing this was when I discovered I have no 4x4 function at the wheel. Transfer case shifts fine and the drive shaft rotates, but I get no power from the differential.

So my final conclusion there is that the noise has too be differential related.