Front drive shaft not fully engaged in 4WD, with truck on stands all 4 wheels do rotate but if i stop front wheels the front drive shaft is not engaged

did you turn the locks on the wheels, or check the fuses

I am thinking that you have a broken/ stripped drive spline on a front axle. If you have reduction gears in the hubs , it may be stripped teeth on a gear. If you jack up one front wheel and turn the drive shaft that wheel should turn. IF so repeat the procedure for the other wheel and if it doesn't turn like the other side then that is the side with the problem

Front axle lock is not engaging, or automatic locking hubs are not activating.

Operationally vacuum shifts the front axle between 2WD and 4WD mode. The GEM ( Generic Electronics Module ) applies ground to the solenoid needed for the mode selected on the switch ( for ESOF ) or when the transfer case shift lever is moved ( MSOF ).The contact plates in the transfer case signal the GEM that the transfer case is engaged, and the Vacuum solenoids need to be activated ( either 2WD to 4WD or 4WD to 2WD ).

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem ( this is what you seem to have, the transfer case is engaged and will drive the front shaft mine was a key on engine off test ).

- If the front drive shaft does not turn :
•With the truck running ( with front wheel back on the ground ), switch from 2WD to 4WD and back again, the vacuum shift motor should move the lever on the front axle.

-- If the vacuum shift motor does not move:
1.Check for vacuum on the lines while shifting between modes. Pink vacuum line is 2WD mode, Light Blue vacuum line is 4WD.
2.Check the fuses for the 4WD system and the solenoids.
3.Check the vacuum lines from the vacuum shift motor to the solenoids on the firewall.

4.Check the vacuum lines from the solenoids to the vacuum reservoir box behind the battery.
5.Check the vacuum reservoir box for cracks.
6.Typical solution is the vacuum lines ( to the solenoids or to the vacuum shift motor ) are cracked or the vacuum shift motor has a cracked diaphragm and has water in it.
7.Could be the shift plate contacts in the transfer case are corroded / not making good contact, and not activating the solenoids.


Usually the case is cracked vacuum lines from the solenoids, the next common is a cracked vacuum reservoir box, and 1 time I have seen the mount for the vacuum disconnect motor broken ( motor floating free so it could not push against the axle )

For the 2002 MY ( this is in the 2006 Section, just want to call it out to others that might find the solution, this does not apply to the 2006 MY )

Operationally vacuum shifts the front axle between 2WD and 4WD mode. The GEM ( Generic Electronics Module ) applies ground to the solenoid needed for the mode selected on the switch ( for ESOF ) or when the transfer case shift lever is moved ( MSOF ).The contact plates in the transfer case signal the GEM that the transfer case is engaged, and the Vacuum solenoids need to be activated ( either 2WD to 4WD or 4WD to 2WD ).


Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
•With the truck running ( with front wheel back on the ground ), switch from 2WD to 4WD and back again, the vacuum shift motor should move the lever on the front axle.
{ picture of shift motor }
-- If the vacuum shift motor does not move:
1.Check for vacuum on the lines while shifting between modes. Pink vacuum line is 2WD mode, Light Blue vacuum line is 4WD.
2.Check the fuses for the 4WD system and the solenoids.
3.Check the vacuum lines from the vacuum shift motor to the solenoids on the firewall.
4.Check the vacuum lines from the solenoids to the vacuum reservoir box behind the battery.
5.Check the vacuum reservoir box for cracks.
6.Typical solution is the vacuum lines ( to the solenoids or to the vacuum shift motor ) are cracked or the vacuum shift motor has a cracked diaphragm and has water in it.
7.Could be the shift plate contacts in the transfer case are corroded / not making good contact, and not activating the solenoids.


- If the front drive shaft turns:
For MSOF systems
-- Check that the shift linkage on the transfer case is moving when the lever is moved from 2W to 4H and 4L.

I am going to take a SWAG that it is 1 of 2 places, both cases the front drive shaft is locking into place.

1. The front vacuum shift motor is not moving ( vacuum or vacuum line issue ).
2. The contact plates for the shift motor are not making contact and are not signalling the GEM that the transfer case state has been changed.

I'd start with the front axle off the ground test, and see which way to go, before you start repalcing items.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

I would suspect that the front drive shaft will not turn, and the issue is with the vacuum shift for the front axle.

The vacuum disconnect motor is on the front axle ( item #10 in the picture below )


Close up of the vacuum disconnect motor ( this is in process of being repaired )


The vacuum solenoids are mounted on the firewall, and take vacuum from the vacuum reservoir to move the shift motor when the GEM ( Generic Electrical Module ) calls for a mode ( 2WD or 4WD ).
The Pink line will have vacuum on it when in 2WD mode, the Light Blue when in 4WD.
Use a vacuum gauge to verify that the vacuum is on the line when selecting the mode.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Engine on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

IF the front dive shaft does not turn, check that the vacuum shift motor on the front axle is operating.


Item #10 in the diagram below.


The vacuum shift motor is vacuum operated ( name says that, I know ) the pink vacuum line is 2WD operation, the Light Blue vacuum line is 4WD operation.
If the motor does not move, you can check the vacuum lines to the vacuum solenoids on the firewall


The other is the vacuum reservoir behind the battery

Auto 4 wd is when the Silverado's PCM takes readings from the front and rear driver shaft and when there is any detection that the speeds are different. The Auto 4WD will activate the transfer case to lock it self to 4wd until the front and rear drive shaft are running at the same given speed of rotation.

Thank you for using fixya

Try removing the front drive shaft from the transfer case to the front differential. With the vehicle on 4 jack stands and the front drive shaft removed attempt to place the vehicle into 4 WD. You may have to start the engine if the vehicle uses a vacuum actuator module to engage the front wheels. When you get an indication that the vehicle is in 4WD try turning the front wheels by hand and see it you hear the same grinding noise .... engage the hubs and try again. Usually if the transfer case is ok then the problem is in the front differential or one of the many universal joints that connect all of the elements together. Check the U joints on the front drive shaft for play as well as the U joints on each of the front wheels.

Lift and support the front of the truck so the front wheels are off the ground.
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.