Autotrac (Auto 4wd) or Controltrac? Is the transfer case engaging and/or the front hubs?
According to a GM tech I know, if 4wd is not engaged on a regular basis the Transfer Case Control Module (TCCM) will lock up. To reboot the TCCM, disconnect the wiring harness at the TCCM or disconnect the battery.
Next check the vacuum lines, I found a few rotten ones on my 2000 Blazer last year. When the 4wd quit completely on the Blazer it turned out to be the vacuum switch that controls the vacuum actuator that engages the front axles. Scroll about 1/3 of the way down the link to see the switch.
Here's another useful link
Good luck and please turn off the caps,