With a short 5 minutes of research, keep checking those grounds, measure for high resistance and test continuity in the circuits from the transfer case harness to the module. I don't have electrical schematics for the first gen, perhaps someone else here could supply those?
I borrowed the below from gm-trucks.com which is a service guide to diagnose U1026.
DTC U1026 Loss of ATC Class 2 Communication:
Circuit Description
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message for State of Health monitoring (Node Alive messages). A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor for that module's "Node Alive" message. Each module on the class 2 serial data circuit which is powered and performing functions that require detection of a communications malfunction is required to send a "Node Alive" message every two seconds. When no message is detected from a learned identification number for five seconds, a DTC U1xxx (where xxx is equal to the three digit identification number) is set.
The Control Module ID Number list provides a method for determining which module is not communicating. A module with an internal class 2 serial data circuit malfunction or which loses power during the current ignition cycle would have a Lost Communication DTC set by other modules. The modules that can communicate will set a DTC indicating the module that cannot communicate. When no message is detected from a learned identification number for five seconds, a DTC U1xxx (where xxx is equal to the three digit identification number) is set.
Control Module
ID Number
ATC
026
BCM
064
EBCM
041
EVO
192
IPC
096
PCM
016
SDM
088
VCM
016
VIU
016
When more than one Loss of Communication DTC is set in either one module of in several modules, diagnose the DTCs in the following order:
Current DTCs before history DTCs unless told otherwise in the diagnostic tables.
The DTC which is reported the most times.
From the lowest number DTC to the highest number DTC.
Conditions for Running the DTC
Voltage supplied to the module is in the normal operating voltage range (approximately 9 to 16 volts).
Diagnostic trouble codes U1300, U1301 and U1304 do not have a current status.
The vehicle power mode (ignition switch position) requires serial data communication to occur.
Conditions for Setting the DTC
A messsage from a learned identification number has not been detected for the past five seconds.
Conditions for Clearing the DTC
A current DTC will clear when a "Node Alive" message from the failed identification number is detected on the class 2 serial data circuit or at the end of the current ignition cycle.
A history DTC will clear upon receipt of a scan tool "Clear DTCs" command.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
A module which loses power during an ignition cycle will cause other module(s) to set Lost Communication DTCs.
A module which loses power during an ignition cycle will cause other module(s) to set Lost Communication DTCs.
The malfunction is due to an open in the class 2 serial data circuit or an open in the module.
The module which was not communicating may have set Lost Communication DTCs for those modules that it was monitoring.
The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
Step
Action
Value(s)
Yes
No
1
Test the battery positive voltage circuit(s) of the module that is not communicating for an open or a short to ground. Refer to Control Module References for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 9
Go to Step 2
2
Turn OFF the ignition.
Test the ground circuit(s) of the module that is not communicating for an open. Refer to Control Module References for the applicable schematic. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 9
Go to Step 3
3
Disconnect the scan tool.
Test the class 2 serial data circuit for continuity. Refer to Testing for Continuity in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 7
Go to Step 4
4
Test the class 2 serial data circuit of the module that is not communicating for an open between the module and pin 2 on the Data Link Connector (DLC). Refer to Circuit Testing and Wiring Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 7
Go to Step 5
5
Inspect for poor connections at the battery positive voltage circuit(s), the ground circuit(s) , and the class 2 serial data circuit of the module that is not communicating. Refer to Testing for Intermittent and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?
--
Go to Step 7
Go to Step 6
6
Replace the module which is not communicating. Refer to Control Module References for the appropriate repair instructions.
Did you complete the replacement?
--
Go to Step 9
--
7
Install a scan tool
Turn ON the ignition leaving the engine OFF.
Select the Display DTCs function for the module which was not communicating.
Does the scan tool display any DTCs which do not begin with a "U"?
--
Go to Control Module References or the applicable Diagnostic System Check
Go to Step 8
8
Use the scan tool in order to clear the DTCs.
Did you complete the action?
--
Go to Step 9
--
9
Select the Display DTCs function for the module(s) which had the Lost Communication with xxx DTC set.
Does the scan tool display any DTCs which do not begin with a "U"?
--
Go to Control Module References or the applicable Diagnostic System Check
Go to Step 10
10
Use the scan tool in order to clear the DTCs.
Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.
Did you complete the action?
--
System OK
