4 Wheel Drive Upgrade Actuator for all 6.5 equipped K series P/U trucks, Suburbans and Tahoes using the upgraded 5 wire actuator.

This is usually the reason why your 4 wheel drive front axle won't engauge. Some select vehicles were factory upgraded from the TLA Thermal Linear Actuator, to a Linear Motor actuator. This is the replacement for that actuator motor. Before ordering, check the harness connector on your actuator. If a 5 wire harness, this is the correct actuator. If a 2 wire harness connector, use the ssd-019. 

There is a switch located on the transfer case that supplies power to this actuator when in 4x4. Check and replace that switch also. Its p/n ssd-016 also in this category.