After spending over $500 on parts and labor at 2 different repair shops, my 2000 GMC Sierra 4x4 was still having an issue with the front axle not engaging or engaged and not coming out of 4 wheel drive. The fix was pretty simple. Found the attached site yesterday and checked the black wire on the wiring harness on the front actuator to make sure it was grounded. It was not and after running a new ground wire everything is working now. I spent over 3 weeks throwing time and money trying to fix this issue. Sure hope this helps someone else....
http://home.roadrunner.com/~roger.hundley/BlobShtml.htm
http://home.roadrunner.com/~roger.hundley/BlobShtml.htm