Get someone to pull it, put it a higher gear... 3rd or 4th and when you get to about 10 or 15mph, dump the clutch. If it is just stuck from sitting that should break it loose.
Or course, it could be the hydraulic clutch slave or master has gone bad and leaked down, in which case, all the above will be a waste of time. I would probably get someone to push in the clutch pedal while I was under the truck to verify that the clutch is being disengaged. Make sure you scotch the wheels before you get under it.