wobble

I have a 1997 chevy 1 ton dually 2 wheel drive. 150,000 miles. I have developed a wobble at about 60 mph. It is in the rear of the truck. I put on new rear tires and it seemed to get worse. The wobble is a back and forth motion and you can see the bed moving in the mirror. It is not a up and down vibration like an out of balance problem. I have checked for tightness on everything from Inner hub nuts to shocks. to lug nuts and can't find any problems.