Originally Posted by fsxbob

So I finally got the 4wd working on the truck, all it took was reading the service manual to learn how the system works. My problem was I didnt realize there were two motors involved in making the 4wd work. You have your transfer case motor and then the front diff locking actuator. I had to replace both because I wasn't sure which was broken.

So common problems with 4wd as I've learned over the past 4 months, hopefully this helps people diagnose their problems a lot quicker than it took me. If i'm incorrect in any of this information, please tell me so i can edit this.

The Switch
The selector switch is known to go bad quite often on these vehicles. Bad solder joints is what i've heard. you can buy Doorman replacements for about $30 or less on Ebay. If you are too cheap for that however you can also sometimes repair the one that you have buy removing it, taking it apart and then reheating all of the solder joints in the back to make sure they are making proper contact. To remove just lower your steering wheel all the way down, put the vehicle in gear (to get the selector out of the way) and then just gently pull the entire dash off. The switch is easy to access at that point. 2/10 on difficulty scale.

You know you switch may be bad if 1. you don't see it illuminated 2. it doesn't respond in anyway to button pushes 3. your 4wd module throws a code, you can only read these codes with a SnapON MODIS or GM diagnostic tool.

The Front Axle lock actuator motor (aka 4wd actuator)
This motor is a linear actuator that locks the front axles in with the driveshaft. It replaces the old method of getting out of your truck to manually lock the axles in place. It is also very exposed under the truck and can easily be damaged and rusted by debris, salt etc. So this one is a common problem but also easy and quick to diagnose. Its a very loud motor so with the truck off you can usually hit the 4wd auto or hi button and hear this motor engage. But if your not sure, lets pull it out.

Go to the front diff, disconnect the power plug and unscrew the actuator. Once the actuator is out, plug it back in and then push the 4wd auto button. This should make just the actuator move. If it doesn't move, check all your electrical connections and check for harness damage. If it looks good, then chances are the motor is bad. They are about $70-90 depending on the source. This is a link to a video showing you how to remove the actuator, it is about a 1/10 on the difficulty scale.
How To Install Replace 4x4 Axle Actuator Chevy Silverado GMC Sierra 1999-06 - 1AAuto.com - YouTube

The transfer case motor
This is an external motor that mounts up the the transfer case. It turns a shaft which engages the front drive shaft and can also put the truck in Hi or Low range. It is also a common problem on these GM 4wd systems.

This one is a bit more difficult to diagnose because it takes significantly more effort to get to the motor. It is located on the transfer case immediately next to the front drive shaft. As well you should not try and actuate this motor when its removed. However it does often throw a code in the 4wd module if its bad, so again SnapON MODIS or GM diagnostic computer will help.

The poor mans way of diagnosing is to just listen for a noise. So hopefully you know what the axle motor sounds like, because this one is must softer sound. So when you hit the 4wd Hi button, first thing the truck will do is lock the front axle. Then it will actuate the transfer case motor to put the truck in gear. So as soon as you hear the front axle motor do its thing, listen for a softer motor spinning, obviously listen near the transfer case.

To replace if needed, you need to drop the front drive shaft to get to the motor. Depending on the amount of rust and your tools, this can be between 1-4 hrs. It took me about 2 hrs laying on the ground with air tools, but my truck is old and fairly rusty. A new motor will set you back about $200, used on ebay typically $70-90. Difficulty scale 5/10 without lift or air tools.
How To Install Replace Four Wheel Drive Transfer Case Shift Motor Chevy Silverado GMC Sierra - YouTube


Other common problems
Grounding or harness issues. People say there is a grounding wire to the chassis below the driver side, so check that and make sure its still there and making contact

Check all of your fuses before buying expensive replacement parts! There are fuses in the cabin and the engine bay that contribute to 4wd, check them all twice.

4wd control module. I didn't mess with this. it is deep in your dashboard and fairly well protected from the elements, but on occasion this will go bad. If nothing works and no codes are being thrown, this may be your problem. Its also an expensive part, so don't mess with it unless the evidence points towards this being broken. If you truck is throwing 4wd codes then its probably not this module.