Hi all. I have a 2000 gmc jimmy. All new vacuum-lines and the vacuum actuator is good. I am in 2hi and I can shift into 4Auto but when I hit 4hi it blinks and i hear a single click then switches to 2hi. When in neutral and shifting to 4low it will flash 4low and the tccm will click multiple times then flash 2hi then all the 4wd lights shut off and the service 4wd light comes on. Then you can turn of the car and right when you start it if you hit 2hi the service 4wd light turns off and ****s to 2hi. This process can be repeated. When the service 4wd light comes on the error code is error C0387. Please help!

 

Have a looked at the encoder on the transfer case..it is weak and pulling too many amps to shift correctly.
Have you also checked out the push button switch?

 

In auto all that happens is the front axle is engaged, the transfer case stays disengaged until the system sees the wheels slipping then the encoder motor engages the clutch inside the TFC to apply power to the front wheels. Until it tries to apply the code will not be thrown.

The code indicates the computer tried to engage the clutch when you hit 4X4 but it didn't happen. Likely the encoder motor has either broken something in the planetary gearset or is reading the position incorrectly. if the lights flash when you press a button, the button is working correctly.