the semi-auto hubs work by vacuum.
you havent said what you have (esof) electronic shift on the fly or manual transfer case control.
there are o-rings and seals (on the automatic esof) systems...if not provided periodic
maint. can leak and prevent the hubs from locking....the hubs can almost always be
locked by hand.
one way to determine what is not working is to slide under the truck and turn the
1/2 shafts @ each wheel...they should lock when the system is engaged...at the same
time when the system is disengaged you should be able to turn the 1/2 shafts by hand.
review the related docs here for some more info -->
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