4wd actuator cable won't hook in axle

the pin may be locked in because of torque wind up from driving on hard surface in 4wd . Check power/ vacuum to actuator as well. Run on grass / mud/gravel/ ice surface as you try to unlock 4wd to allow the torque wind up to disappear.

Check the 4x4 wiring ground (directly behind the left front tire, mounted to the body). If the front axle actuator is still good, this is usually the source of frustration. Put some RTV Gasket sealer on the nut and bolt after you replace them to help keep it from corroding as fast.

on the 1999 silverado the transfer case has an actuator motor to engage from high to low range ,for 4wd there is an actuator motor on the front axle, had same problem in my 98 changed and works perfect

Sounds like you have the actuator switch set to tight. You might want to try to adjust the switch so it has a little room between the switch and the shifter. Hope this helps.

i've been told it was the 4wd actuator located on the passenger side near were the cv joint goes in to the front axle a good test is to floor jack it up so both wheels are off the ground and have someone on the other side , put in nutrual turn both wheels one way at same time and the 4wd drive shaft should turn if not its your actuator, costs about $86 for anew actuator and takes about 5 mins to change once you have the tire off

On the full size, the front actuator is electric, not vacuum.

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

If I were you, I would buy a new heated actuator. They are available at most auto parts stores, and are about $90. They are also on eBay for about $55 plus shipping. These are very easy to change - just thread out the old one, and thread in a new one.

Good luck, That actuator is usually the problem 90% of time.

Found this repair link on fixya, it's great I just bought a manual. Try it :- Best Manuals

have you checked to ensure that the actuator is working? if the actuator isn't working then the ring gear wont engage and won't turn the front shafts. try turning your key to accessory and put the truck in 4wd

Sounds like your front actuator in the axle is not working..What you can do for now instead of buying a new one is remove the front actuator.It is a threaded in unit.Take a crescent wrench and remove it.Next find a big nut or bolt to put inside of it.Once you have it off you can jack the front end up put your finger inside of the hole where actuator goes.Rotate the front tires back and forth and till you can push the tab inside in.That will lock the front end .And the reason your putting the bolt or nut in there is to take up the space the actuator would normally would Push in..Remember your front axle is always going to be lock and you will still be able to unlock it with the push button.After you put the bolt in the hole screw the actuator back in.This will hold the bolt and keep your axle locked.........

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589

there is a vacuum solonoid on the firewall in the middle with 2 vac lines and a 2 wire plug
thats ur problem
push the button and smack it with the handle of a screwdriver should work