Here's a trouble shooting guide for you with repair parts listed http://www.4x4posi-lok.com/PDFs/troubleshootguide.pdf sounds like its common on these transfer cases, Once you download it scroll down to get to your year make and model. Napa has rebuilt transfer case motors available for around $155 online
Check out and read this page here and he will answer your question faster than it can be explained to your here in this forum. This is a common failure on electronic shift GM 4X4's. This page goes into great detail on repairing the problem.