Brad, the newer Fords have a vacuum controlled diaphram in each hub, I think Ford calls them seals, that can fail at the most in-opportune times. When this happens, you can still lock the hubs manually. I didn't know this until after I was stuck in the sand a couple of months ago
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On Edit - BTW, for those with 4x4 that don't already know, you need to engage the 4x4 about once a month and drive a couple of miles, even if the roads are dry. This will insure all the bearings and gears get lubed, and moved. The roller bearings in the front differential will wear grooves in the bearing races just from sitting in the same place all the time. Then, when you DO need it, the vibration will make you think the front axle is about to come apart, and it is.