Looks like when someone converted it from manual to automatic, they obviously installed the automatic steering column but might have left the park/neutral switch out of the picture. On a manual, there's a clutch safety switch that performs the similar function as park/neutral (won't let the starter engage unless you have the clutch depressed) so that is likely jumpered so your truck will start.
You'll see the slot for the park/neutral switch on top of the steering column tube. The wiring and associated colors may not match for clutch safety vs. park/neutral safety so you may need to improvise - can't recall for sure on that but the factory service manual wiring diagrams can help there. You'll find those manuals available for free download in the "Engines" section. On a manual, the reverse switch is actually on the transmission, not on the column, so you may need to locate the wire colors under the dash and splice on an appropriate connector for the column-based switch.
Richard