After driving the truck around for five minutes or so it ran out of fuel as usual, but when I filled it back up again and started the engine, the engine started but the wheels don't move. The engine will rev up when I hit the throttle but no wheel movement. I can spin each wheel and the opposite wheel spins. I didn't see any thing bent or bound up.

What else could it be?

Thanks

You may have stripped a spur gear. Or, it could be bad in just one spot.

start it up and throttle it lightly,
check to see if the pinions (smaller gears) are turning. If no, then likely the clutch.
If YES then check pinions(larger gears),
If the are not turning, then check if either the spurs or pinions are stripped.
If spurs and pinions are good then check your driveline, But if the wheels turn eachother, then I would suspect, as alvinl says, a stripped pinion, or some problem with the clutch.

check if there is any debris in the spurs

I can spin the main gears next to the clutch clockwise easily with my fingers but I can't spin it the opposite way.

Do you think it's the clutch, differential or something else?