depends on you truck, for some I think it was an option. If you have the switch on the dash you have ESOF
hubs Electronic shift on the fly hubs where vacuum lock them in then you have the manual way to lock them on the outside.
I have one of my hubs that leaks it won't hold any vacuum so I just lock my hubs. One of these times where the weather is nice it will rebuild it.
this is how you can test your hubs. Also listen for the vacuum pump to turn on right when you turn your key over.
Hublock Vacuum System Leak Test
NOTE: This procedure requires the use of a calibrated vacuum gauge that is accurate to within a minimum of 0.1 in-Hg. It is recommended the Pressure Vacuum Module 105-R0099 or equivalent be used.
Make sure hublocks are in the AUTO position.
Lift and support the vehicle off the ground until the front tires can be spun freely. Refer to Section 100-02
NOTE: The engine is required to be at idle during the following steps to supply the vacuum required for the hublocks to engage and disengage.
Start the engine and allow to idle.
Turn the mode select switch (MSS) to the 2WD position and wait 15 seconds. If switching from 4X4
mode, wait an additional 45 seconds.
Disconnect the vacuum line at the right front knuckle and install a vacuum gauge on the line.
Disconnect the vacuum line at the left front knuckle and plug the line.
NOTE: Normal operation applies engagement vacuum for 45 seconds and then drops to 0 in-Hg.
Turn the MSS to 4X4
and observe the vacuum reading. The vacuum reading should remain at or above 10 in-Hg for 45 seconds.
NOTE: Normal operation applies regulated disengagement vacuum for 15 seconds and then drops to 0 in-Hg.
Turn the MSS to 2WD and observe the vacuum reading. The vacuum reading should remain between 5.8 and 7.3 in-Hg for 15 seconds.
Remove the vacuum gauge and connect the vacuum line at the right front knuckle. Turn the MSS to 4X4
NOTE: The right front axle shaft and universal joint should turn.
Wait one minute then rotate the right front tire one revolution forward and one revolution backward while observing the right front axle shaft and universal joint. Turn the MSS to 2WD.
NOTE: The right front axle shaft and universal joint should not turn.
Wait one minute then rotate the right front tire one revolution forward and one revolution backward while observing the right front axle shaft and universal joint.
Install the vacuum pump and gauge line on the knuckle and pump to 20 in-Hg. If the vacuum drop is not equal to 0.5 in-Hg in 30 seconds, Go To Pinpoint Test G for further diagnosis of the hublock system.
Repeat steps 1 through 13 for the LH hublock.