Rear brakes going out before front brakes on 2008 Chevrolet Silverado 1500

On my last oil change, my dealer gave my rear brake pads a need replacement soon with only 4mm remaining on the rear brakes. My front brakes were given 5mm. It is only front disk. The vehicle is a 2008 Chev Silverado 1500 4x4. The mileage was just over 15000 on the oil change. Is it normal for the rear brakes to go out before the front brakes? I was told I should get 40,000+ out of the rear brakes. Is this true? I do not do any towing but there is a lot of hills. I can also feel a sligh lock up on the rear brakes when going from drive to reverse and vice versa. Any help would be great

The rear brake shoes on your 2008 Chevrolet Silverado may be of different thickness begin with. The front brake shoe lining may be thicker than the rear shoe lining by design. Measuring the thinner shoe only can result in premature replacement of the rear shoes. When new the thinner rear shoe may not be much more that 5mm to begin with. I would recommend careful inspection of your rear brakes during your next service visit. If you can, please try and better describe the "lock up" you are experiencing when shifting from drive to reverse.