You have what is called an "On Demand" AWD system. It is described by ALLDATA thusly:
"The second category is an on-demand AWD. This type of AWD basically
delivers torque only to a primary driving axle unless reduced traction
is experienced. At that point the system electronically or mechanically
will apply torque to the other axle. Depending on the type of system
this can provide up to 100% of the torque to the axle with traction.
These transfer cases use an electronically actuated clutch pack a hydraulically actuated clutch pack or a viscous coupling to allow for a difference in speed between the front and rear axles.
An example of an electronically controlled version of the On-Demand AWD
is in the Smart Trak system in the 2003 Bravada. An example of a
hydraulically operated On-Demand AWD is the Versa Trak system in the
Aztek while a viscous coupling is used in the Vibe. The RPO code for
the Bravada is NP4 (NVG1 26)."
So it seems that your clutch pack is not being activated electronically as it should. This is where you need to start your diagnosis IMO.
- The diagnostic trouble codes are displayed on the three transfer
case shift select buttons. The shift select buttons are located on the
instrument panel when the connector pin 13 on the data link cable is
grounded, and the ignition switch has been OFF for at least five
seconds prior to positioning the ignition switch to run the shift
select buttons will blink various times together in order to indicate a
diagnostic trouble code from 1 to 4.
Ensure the ignition switch is positioned to OFF for at least 6 seconds.
||Position the ignition switch to OFF.
||Connect pin 13 on the data link cable to a vehicle ground source.
The data link connector is located in the cab under the instrument panel on the drivers side.
||Position the ignition switch to RUN.
||Note the shift select buttons for blinking codes. Refer to Perform a Diagnostic On the TCCM.
- If the shift select buttons all blink one time and stop, and do not continue to blink, no fault codes are stored in the TCCM.
- The transfer case shift select buttons will blink in order to
identify any stored DTC. If only one code is stored in the TCCM memory,
that code will blink repeatedly with a three second delay between
blinking sequences. If more than one code is stored, the first code
will blink once, then after a three seconds delay, the next code will
blink. This sequence will continue until pin 13 is no longer grounded.
- When reading the diagnostic trouble codes, the number of shift select buttons blinks will indicate the code number.