My 2009 F-350 4x4 has a front wheel option of Auto or Lock ; I think I understand the Auto option but under what circumstances would I lock the hubs and is it safe to leave them in the Lock mode when traveling on the highway?

 

The manual lock option is basically a backup in case the hubs don't automatically lock. Theoretically you shouldn't ever have to use it, but it's nice to have just in case. If you knew you were going to be using 4wd, you could manually lock the hubs ahead of time if you wanted to. It won't hurt anything to drive around with the hubs locked in, although it is unnecessary wear on some driveline parts and maybe a very slight hit on gas mileage.

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If you leave them locked it'll just be like the D•dge.

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Dodge isn't the only one. In the early 2000's, the Rangers went to locked front ends. Jeeps, too.
Your rear drivetrain turns all the time; it doesn't hurt the truck to be driven with the hubs locked in, but it does create more drag (slightly increased fuel consumption), and the front parts wear when they are turning.
Saying that, it is a good idea to drive the truck once a month or so with the hubs locked in, so that oil gets moved around in the pumpkin and transfer case, and grease gets worked in the front u-joints.

 

In my 2006 F350, it stopped engaging in 4wd when the hubs were on "auto" years ago. I didn't bother getting it fixed. At the beginning of each winter, i just put the front hubs on "lock" for the whole winter. That way, when i actually need to engage the 4wd from inside, i know that it will work.

 

Thanks everyone.