2002 suburban stuck in 4hi

[Chevyman5197]

I have a 2002 and its stuck in 4 hi.I can hear it shift then it goes right back to where it was.I have replaced buttons in dash,transfer case computer and encoder motor on case.I had encoder motor off to see what it was doing and when I push 2 hi and watch it turns then turns back to where it was.I checked codes and had co323,co327,and co387.

 

[zora]

try following these diagnostic steps