I grew up on a farm, so dogs always went in the back. You never knew what they'd get into and you didn't want whatever it was inside. Also, I usually had a full cab moving workers around. Dogs learned early that, if they stayed in the bed and off the toolboxes, they'd =stay= in the bed. They started getting up on the boxes, they could very easily fall out. Field speeds, not highway speeds, but a 4' fall learned 'em quick. When traveling more than a couple miles, they got short leashes snapped to the GN hitch, enough to let them stand, but not enough to let them jump near the bed rails. Now, if I ever get another dog, it'll be small enough to ride inside =and= I have a crew cab, so more space.
Lyle