i had a simular problem with my RPM gauge, one day started truck and the RPM went to redline at idle, restarted truck and the needle stuck at 4500.
time to call dealer, they replaced the whole cluster under warranty, $650 or so, told me the new cluster would have to be VIN coded to my truck.
here is what "mygmlink" said after service repairs.....
CLUSTER ASSEMBLY, INSTRUMENT PANEL - R&R OR REPLACE, INSTRUMENT CLUSTER-ASSEMBLY RETURN TO AC DELCO ESC