I have a '98 Blazer 4dr 4.3L Vin.W. Early on this year the my 3 button 4WD stopped working. When I would try to shift to 4 hi or 4 low it would flash and go back to 2 hi. TCCM right? Replaced it. Still did it. Checked all my vacume lines. I mean took them off put a gulf tee in one end put them under water and blew air in them. GOOD. When I manualy suply air to the actuator it works. and will engage the front axle. Changed the fluid in the trans case twice since Jan. Now my 3 button 4WD switch doesn't light up at all. Someone told me the tans case actuator. That's just an expensive part to just "try". So any thoughts? Anyone?

 

The TCCM is an expensive part to just change. The transfer case vacuum switch is about $20 and goes very frequently. Also the dash switch goes a lot. Again $20. My money would be on the transfer vacuum switch.
Here is a very good read on this subject.
https://blazerforum.com/forum/tech-a...ill-help-7180/

When the truck is in park and you push the 4hi do you hear anything at all?

http://www.rockauto.com/catalog/more...568&cc=1314133

http://www.rockauto.com/catalog/more...130&cc=1314133

 

When mine quit working I pulled the dash switch and noticed the solder was broke where the switch is soldered into the circuit board. Re-soldered switch back in and it's good as new.
Had same symptoms as you.