excessive rocking and clanking during spin cycle

top loading Kenmore 90 series washing machine tub rocks and bangs during the spin cycle but when I hold or put pressure on the outer edge of the tube it the clanking stops but the tub still vibrates. I first made sure the load is balanced and the washer is level with the same effect. Next, I tried the spin cycle with an empty tub. It spins ok till at full speed and then the rocking and banging starts. While watching it I observed that the tub seems to lean back and bang against the back of the frame so when I put pressure on the front of the tub the banging stops. It still gives a strong vibration workout. Next, when stopped and I push down on the agitator there is no bounce but if I rock the tub it returns.