4 Wheel drive problem

First snow of the year today.  Popped the truck (2001 F150 SuperCrew) into 4Hi to get into the sub division.  Had a heck of a time getting up the hills.  I was able to determine that the the front wheel were not engaging.  Tried 4Lo after I got home still nothing.
 
I had my daughter drive while checked underneath and I the front axel was engaged and spinning when in 4HI.  I had new tires put on about 3 months ago.  Could they have messed something up or are my Hubs shot?
 
If it is Hubs, how much will that set me back.  If they messed something up, what would I look for to rectify the problem.
 
Thanks

Need to check the vacuum system to the hubs for proper operation before condemning the hubs.

Agreed.

Thanks, can you guys give me a knudge in how to check this.  I typically just to the normal maintenance stuff like brakes and oil.

There should be a vacuum line from the front pumpkin/axle that you can follow up to the air solenoid that is used to activate the 4WD.  I'd pull that out to see if it needs to be cleaned.
 
If you are willing to shell out $16 or so, you can get a Haynes manual for your vehicle that will theel you all you need to know about checking and changing it.  You can get them at any Autozone, Advanced, ect.