This makes particular sense for a pickup. When unloaded the truck's weight balance significantly skewed toward the front, allowing for less traction in the back

This makes particular sense for a pickup. When unloaded the truck's weight balance significantly skewed toward the front, allowing for less traction in the back, and so you need to rely on the front wheels more for braking. When you're loaded, that's sitting more-or-less over the rear axle, increasing traction there. So you can afford to rely more on the rear (driven) wheels with engine braking in addition to the conventional brake balance that's biased to the front.