I would try a better penetrating oil like Kroil or PB Blaster. WD is pretty weak when compared to these. Putting the truck in high gear and rocking it back and forth will do the same as using the prybar.
You might pull the oil pan to see if any of the bearings are seized. You could also remove the bearing caps from the connecting rods one at a time and try to turn the motor over with one rod unbolted. This will at least tell you which piston is stuck.
Phil
