sounds like a allen screw came out of your front or rear drive shaft or the (if its the pinion gear on the engine not turning) the clutch springs broke. i would start either at the engine cb or the front or rear drives. if a allen unthreaded itself the center diff will give all the power to the loose axle and the truck wont go anywhere, or if the clutch is broken on the engine the motor will spin and not move the gears. and since the engine was pulled once before i would start their at the clutch bell and check if the springs are broken.