The springs, brakes and rear axle are heavier on the 3500. Look at the end of the rear axle where the wheel fits on it. A 2500, like my B, has a semi floating rear axle. The axle rides on a bearing at the end of the axle tube. A 3500 will have a full floating axle. The axle doesn't carry any weight. All the weight is carried by a huge bearing assembly and the axle just turns the wheel. It's marked by a large rough bolt together assembly sticking through the wheel. You see these on larger trucks. You can even unbolt the end of the axle and slide it out with the truck sitting on its wheels. The lighter axles you have to take apart inside. If you're going to carry a lot of weight, you want the heavy duty axle assembly.