Replacing air conditioning belt

Can't be to sure with yours. It maybe the switch. On my 2002, I found I had a Transfer Case Control Module issue. Shopped around and found one on Amazon for half the price the local box stores wanted. I would Google this before starting to buy a bunch of parts.

This video should help. It's a 2000 Silverado but is probably the same as yours.
2000 Chevy Silverado 1500 front brake pads rotors replace

check the four wheel drive fuse.if fuse is good it is probably the tccm.

never knew 4x4 had a chain inside the box ??

It sounds like a faulty sensor wire on one of the calipers, I'd check the front first because they move around more than the rear. If the brakes have been replaced, sometimes the sensor wires don't get instaaled right and they either don't make contact or they get broken, difficult to put in unless you have experince with the new brake systems.

towing any 4 wheel drive is not recommended unless you remove the drive shafts, better to trailer it

I had a similar problem with the Service 4 Wheel Drive light which came on intermittently then became more consistent in the message center display. It would go into 4 wheel drive if the message center did not display the Service 4 Wheel Drive message in the message center, but not when the message was displayed. I found the transfer case switch module in the dash was defective, and it has been working with no other problems for three years.

try selecting from nutreul or less then 3km

Air Conditioning Belt

1. Remove the accessory drive belt.
2. Raise and suitably support the vehicle.
3. Remove the engine shield bolts.
4. Remove the engine shield.
5. Install a ratchet into the air conditioning (A/C) belt tensioner adapter opening.
6. Rotate the A/C belt tensioner clockwise in order to relieve tension on the belt.
7. Remove the A/C belt from the pulleys.
8. Slowly release the tension on the A/C belt tensioner.
9. Remove the ratchet from the A/C belt tensioner.
10. Clean and inspect the belt surfaces of all the pulleys.
    
    Fig. Location of air conditioning belt tensioner-4.8L, 5.3L, 6.0L Engine
    

Replace the push button selector assembly. Common failures. If that isnt the problem then the encoder motor on transfer case has failed. Try switch first.