Ok, I have a 2000 gmc
jimmy, np8 transfer case.the 4wd quit shifting some time ago. The lights on the 4wd selector buttons would flash sometimes. I replaced the vacuum actuator, I just replaced the transfer case shift motor and it still isn't shifting. The service 4wd light comes on. The vacuum actuator has vacuum and will pull the front axle into 4wd. What's wrong with my jimmy??

 

Have you checked the connections on the TCCM ?, they can get corroded and cause problems . It may help also if you read the service 4x4 code, ...can you hear the TCCM clicking when you try to select 4x4 ?

 

Ditto on the TCCM; they can fail. When going bad you can sometimes hear them clicking multiple times.