4x4 not working

[petzl21]

OK, i am new to Ford's, and just picked up an 03 F350 SRW with the 7.3 and an auto, and i love it.

That being said, i am having some issues with it that i have been addressing and the latest is the 4wd. When i turn the know to 4hi, the light on the dash goes on, the front driveshaft is spinning, but only the rear tires are spinning.

I am assuming it is the hubs. Strange part is that i plowed with it the previous day and the 4x4 worked fine ( im sure it worked because there is no way i would have been able to back up that hill in 2wd). These hubs are only auto locking, right? They look like a dial, but do they turn also (so a manual option if the vacuum is not working)?

So, what are some things to check? The relay for the vacuum has power, and when i pull the lines i hear vacuum. is there a seal in the hub i could grease? If so, what kind of grease should i put on it? Or is it a better bet to just replace the hubs?

Thanks for any advice! I want to get this taken care of before winter (although we just got 10inches here in ct)!