Hy Gooberdog, welcome, the door lock only on the right hand door was supposedly a safety thing, you could push the inside door handle on the left door up or down (I can't remember) and that would lock the left door, then you could slide across the seat and exit the truck through the right hand door, now you are standing safely on the sidewalk while you get your keys out of your pocket and lock the right hand door, hope that helps.
3B, as usual, has it right: " . . lock the left door, then you could slide across the seat and exit the truck through the right hand door, now you are standing safely on the sidewalk . ."
The handle up or down (depending on the year) locks the door from the inside.
