The brakes are working as designed. The front brakes are bigger and designed to do most of the stopping when the truck is loaded. If the front brakes went to work with the box empty the truck would go sideways in an instant, especially on a wet road. The proportioning valve is set to have the rears do most of the work when the truck is unloaded to keep it in a straight line.
If you towed most of the time the fronts would wear out first, but since we're empty or lightly loaded most of the time the rears wear out first.
Regarding the rears, if one side was down to bare metal and the other rear still had meat on the pad, then you may have a dragging caliper on that side. be sure to remove and lube the caliper slide pins as part of the job. Also, be sure to put the pins back in the right spots, they are often not interchangeable top to bottom and can cause brake wear like this.