With the engine off, it should go into at least one of the gears if it's just the clutch. I can't imagine what inside the tranny other than it being a complete rust ball that would cause is not to go into any gear. You may have to roll the truck a bit to get the gears to line up. Shouldn't take much movement.
More likely the clutch disk is stuck to the flywheel. If you can get it into high gear, have someone step on the clutch and rock the truck back and forth, which may break the disk free. Or with it in high gear, apply the parking brake, block the wheels, and crank it with the starter (also with the clutch depressed).
