Alright, get under the truck with it on jack stands and in neutral, and spin the front drive shaft by hand. The axle shafts should be spinning (atleast one of them as it's got an open diff). If the axle shafts are not spinning, you've got
A) a smoked pinion/ring gear - you should hear some noise if this is the case...and its very hard to break this before something else
B) a broken axle shaft. If an axle shaft is broken up by the diff, then the power will get transfered to the broken shaft. Again, you may be able to hear some noise, and then again you might not - it depends on what side broke as only one is held in with a c clip - the other is simply slid in. I've never seen these axles break there though - they almost always break where the U joints are, or close to them....
C) possible that the splines on the pinion gear are smoked, allowing the drive shaft to spin over it - but that's highly unlikely.
If the axle shafts are spinning, then your hubs are more than likely bad if the wheels (or one of them) isn't spinning. If one of the wheels spins, have someone hold it stationary while you spin the drive shaft. The other wheel should spin as the power will be sent to it - if it doesnt then you've got a bad hub.......