[jsbihn]

I had a feeling there was a computer issue based on the fact that usually if there is an issue with that much involved in the Vette it has to be a bad sensor, wire, or module.

Did you have them clear the codes to see if they were active or history codes?
I always do that because active codes will mean a broken something. The codes will not clear. history codes will give u issues but if cleared and they never come back then it was probably a one time drop in voltage or something.

(ie. after I had the s/c installed on the Vette my car would constantly drop into "limp mode" for no reason. Sometimes if i floor it, other times if i dont. Sometime if i just drive it normal.... there seemed to be nothing to cause it. Since the installed was in a far away state I couldnt easily bring it to them, so 1500 dollars later my tech finds it is the crimp connector used for the TB. turns out these computers are so sensative that the slightest drop in voltage will through a code. After a quick clip clip and solder, the car has been strong ever since.
I assume the Hummer is the same way

HT, do you think the ebcm or the abs module is causing the issue with his inability to shift his 4wd?