First. The ground wires on these old trucks go bad from the inside out.. they will show continuity but will not carry the load..
The starting circuit runs through the gear selection switch on the side of the trans. The start "relay" part goes bad. Nothing else wrong with the switch. And they are pricy to replace.
There is a write up on yotatech on how to bypass the switch and run a new relay. How ever after the bypass. The engine will start in any gear. best of luck
