Wow this is getting confusing! All I've ever heard/read is never be the owner of a blazer past 100,000 miles, but I couldn't let her go! I just made it over that mark, (100,660 Miles), and this is how it repays me haha.

Okay. Thanks for bearing with me. So as I understand it, when I press that oh so frustrating "4WD" button, there is a wire that runs to the Transfer Case Control Module (TCCM) located somewhere behind the passenger side kick panel.

[I am still hearing a 'clunk' after pressing that button, even with all the vacuum hoses disconnected.]

Then, the TCCM via wire sends a signal to the Transfer Case Motor (TCM), telling it to allow vacuum to the transfer case switch, which then opens and allows vacuum to the actuator beneath the battery tray, causing the cable from the actuator to be pulled, engaging the passenger side CV axle coupler to be engaged to the front drive shaft (which is always connected to the front left tire [that bit of info has contradicted itself in many of my searches]). This is what I've been able to gather reading all sorts of threads and google search results. Sound right? I'm just trying to understand how this all works.

I forgot to mention that when I press 4WD, the light switches to the 4WD button and stays lit, much like what you experienced aa? My TCCM apparently isn't realizing I'm not in 4WD either, which might be due to a bad sensor on the axle? What was your problem?

EDIT: So I could find the TCCM, disconnect it, and drive it to make sure I don't hear any sounds without any problems, correct? This would help me narrow down possibilities.

 

I agree with aa21830 on the encoder motor shifting to 4HI. Not sure why, maybe it's only on the 4 button systems, they will shift back to 2HI if the axle engagement switch is not satisfied. Without vacuum to the actuator, the switch obviously will not be satisfied. It's only a spring loaded, normally open on/off switch, nothing fancy. I suppose it's possible that it's stuck ON causing the TCCM to "think" it's in 4WD and leave the transfer case in 4WD... who knows, have to use an ohm meter to check it.

Without vacuum to the actuator, and the humming noise going away when you select 4WD is puzzling. aa21830 might be onto something when the driveshaft is powered... possibly a worn slip joint or U joint on the front driveshaft, or possibly a worn output shaft bearing in the transfer case. The shaft rotates whenever the vehicle is in motion, it's just not powered by the transfer case in 2WD, but it does rotate. Might be just enough slop somewhere to create the noise. Raise the front wheels off the ground and check for movement, (wear) in the front driveshaft and yokes.

When you select 4WD HI on the dash switch, the TCCM commands the encoder motor to perform a "mode shift", (2WD HI to 4WD HI). This applies power from the transmission to the transfer case output shaft, (inside the transfer case). When the linkage inside the transfer case moves during the shift, it pushes the spring loaded ball on the vacuum switch. The vacuum switch applies engine manifold vacuum to the axle actuator under the battery. The actuator pulls a cable that runs to the right front axle tube. Inside the axle tube there is a clutch mechanism that connects the inner axle shaft, (not the half shaft) to the differential, and the differential becomes "live". This can all be done "on the fly". Selecting 4WD HI to 4WD LO, the vehicle must be stopped, or barely rolling, in neutral. This changes the gear ratio in the transfer case and is called a "range shift".

I think I'd check the drive shaft for a problem somewhere causing the humming noise.

 

Well I have finally worked it all out-- bad vacuum switch and Transfer Case input shaft seal. I replaced the vacuum switch, blew out the lines to/from the switch and actuator, capped/plugged the HVAC vacuum lines (default is mixed defrost and floor heat so I'm fine ) and took it to a friendly 4WD shop that replaced the input shaft seal----the transfer case had 2 extra qts of tranny fluid in it! I again thank all of you for the spot-on advice