I have looked through other threads and haven't seen my exact issue. I have 99 Blazer with button 4wd. When you push 4 HI button, it sometimes goes into 4wd on first try and sometimes not. When it doesn't, you can hear some clicking and the lights on the 2wd and 4wd buttons flash, and it goes back into 2wd. If it does go into 4Hi, then the same thing happens trying to get back into 2wd. But, if you push 4 AUTO then 2WD, all the lights flash then go out, but it does go into 2wd.

I ran diagnostic on it and it throws code C0323 - T Case Lock Circuit Low. I checked the vacuum solenoid under the battery and everything there works fine.

From what I can see in other posts, it seems that it is either the TCCM or the module on the transfer case. Any way to tell from the symptoms which it is?

 

Double check the condition/connection of the solenoid's vacuum hose(s). My my '02, a vac hose had dry rotted causing a similar symptom. The '02's solenoid is located on the firewall.