How much the rear wheels push, or pull, depends directly on how much hydrostatic pressure you are building. In first gear, you build the least pressure. Each gear you move up increases hydrostatic pressure to the back. When the front wheels pull hard, the back axle pushes more. The reason your back wheels spin out in reverse, and not forward, is because of the direction of torque on the front axle. When going forward, the back torque, from the front wheels, increase weight on the back axle thus increasing traction. When you go in reverse, the opposite happens, the front axle tries to lift the back axle off of the ground. When this happens, you spin out. If this happens a lot to you, I would suggest larger tires in the rear. This will add weight and increase traction on the rear axle or a use smaller header on the front.