Before you pull the head, you need to get under the truck, pull the flywheel pan, and with a really big screwdriver or a flywheel turning tool, you need to try to gently budge the motor by prying a bit on the flywheel to move to motor around just a bit, get it started. It needs just a tiny bit of a jolt to brake it free. If you can’t move it at all, then I do think you might have to pull the head.
