Have someone climb under the truck while you push the clutch to see if the slave cylinder is actually moving. It might be jammed in the out position, forcing the clutch to disengage all the time. Try removing the slave cylinder from the side of the transmission altogether, this should allow the clutch disk to engage. Make sure you're not going to hit anything if the truck moves forward of course.
If it still doesn't work, you misaligned something with the clutch disk/pressure plate and I'm afraid you're stuck pulling the transmission back out.