that happened to mine and i removed the vacume actuated shift pod on my front diff. take it off and lube the moving shift fork back and forth. reinstall and it should be fine. and f.y.i. these trucks dont have auto hubs or locking hubs out on the end of the axles like the older fords. when u shift into 4x4 your transfercase gives power to your front diff and the vacume pod on the diff engages the axles.
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Like 7700 says, these trucks have no locking hubs. If the axle pod is good, right behind the battery is the two solenoids that control the vacuum to the front differential. You can check those vacuum lines for cracks or breaks at those ports. Good luck
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