Here is also a TSB for this problem.
Drivetrain - Slow Or No Engagement of Front Axle
File In Section: 4 Drive Axle
Bulletin No.: 76-43-01A
Date: September, 1998
Slow or no Engagement of Front Axle when 4WD is selected (Replace Front Axle Actuator)
1988-97 Chevrolet and GMC K Models
excluding 1988-93 Models over 8500 # XXXXX
This bulletin is being revised to add information to the Service Procedure, add a new part number, and include schematics. Please discard Corporate Bulletin Number 76-43-01 (SectionXXXXXAxle).
Some owners may comment that the colder the outside temperature is, the longer it takes for the front axle to engage whenXXXXXis selected, or the front axle does not engage.
The front axle engagement actuator is a thermally activated component. Because of this characteristic, the time required for the actuator to completeXXXXXengagement is extended as the temperature drops.
Replace the front axle actuator and include the applicable wiring harness kit. Refer to the Parts Information in order to determine which wiring harness kit is needed.
1996 model trucks with a build date on or after January 2, 1996 are considered as "961". If the build date of the truck is unknown, verify the 3 character "axle code" on the front axle label in order to determine which wire harness to order.
This new actuator uses a motor to engage the front axle and is not affected by temperature. On 1988-93 models with a GVW rating over 8500$, the new actuator cannot be installed and the thermal actuator will remain the service part.