Actually that’s normal. I believe it has something to do with what the rear pads are made from I often find they have a different ‘blend’ than the fronts, but your situation is quite common in my experience. I’m sure they’re not doing the majority of the braking if they were your truck wouldn’t stop worth a darn, if the stopping force seems normal I would move on and just expect the rear pads to wear out quicker than the fronts.