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4 wheel drive vehicles operate by connecting all four wheels to the drive train instead of the usual 2. This is elementary, of course, but it seems to escape the grasp of many people. As evidence I offer that many four wheel drive trucks and cars that come to my place of business have mismatched size and types of tires and even wheel sizes. Such a practice can place a lot of strain on the rest of the drive train because some wheels are trying to go further than others. In slippery going this can cause slippage and dangerous loss of traction. A common result is difficulty in shifting out of 4 wheel drive back to 2 wheel drive because of a buildup of pressure from differing wheel sizes.
Another item that comes to mind is the "lockout hubs" that most modern 4x4's come with from the factory. So many times you see a four wheel drive vehicle experience a little traction difficulty, and the driver hops out to manually turn the hub to "lock", while like as not whomever is riding shotgun will hop out for the same drill on the passenger side. After the need for the ultimate in traction & go power has passed, they both will repeat the familiar drill to turn the hubs back to "free".
So now the entire drive train for the front wheels will lay immobile collecting salt, water, sand, etc., and rusting solid while the owner/driver is thinking of the fuel being saved by not expending the horsepower to turn these parts unnecessarily. This is the biggest reason these front wheel drive parts such as universal joints and splines need to be replaced far more often than the corresponding parts for the rear wheels.
The factory literature will usually cite a fuel savings of somewhere in the vicinity of 6% in highway driving, but most driving of a 4x4 in winter is probably not so much highway use as local use. However, the cost of rebuilding the front driveshafts and axle joints can far exceed the meager savings in fuel. (Actually, I shouldn't even mention this, as it is quite a profitable job.)
My recommendation (for the Northern New England area): On or about October 15th, turn those hubs to "lock" and leave them there, (unless you are about to embark on a highway trip to southern regions on a dry road), and leave them in "lock" until April 15th. If you should happen to think of it during the season intervening, especially if running in the rain, flip the lever or whatever control they gave you into "4x4" position to give these parts a little exercise and redistribute the lubrication that you have hopefully given them, and fling the water off any exposed parts.
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