I find your Envoy issue truely curious. In particular because I also own an 02 GMT360. Your description of what happens with the security light and unknown driver message I expect to see with a hosed serial data line. But the cluster gauges going full scale upward is bizzare. A cluster reset drives the gauges down to the max. I have never seen nor can I duplicate an upswing on my frankenstein benchtop of PCM, BCM, Cluster, Ignition switch and whatever other random modules I throw on there.
The Envoy connects with no more than 3 wires, power, ground, and serial data. If the DLC cable and obd2 adapter are sound that points to a bad VCI I would have to guess