Automatic 4wd Problem

[jon99z71]

I have a 1999 GMC Sierra 1500 Z71 with automatic 4wd. The "SERVICE 4WD" is desplayed in the information center. When I connected the code reader to the truck I got code "U1026 LOSS OF CLASS 2 COMM WITH ATC". The 4wd does not function at all and no lights are on in any of the push buttons on the selector switch. The only time I get any lights on in them is when I start the truck, they all light for a split second and then they stay off. I have replaced the selector switch with a new GM one before I put the code reader on the truck because I read several articles from guys having what sounded like the same problem and replacing the selector switch solved it. In my case it sounds like I have more than one problem, I can't access the 4wd module using the code reader because of the loss of communication between it and the PCM and the 4wd will not function at all.

Any advice would be greatly appreciated.

Thanks