drive shaft out of balance or torque converter problems

You have to determine whether the vibration is in the front suspension or from the drivetrain. If you tap the brake, the torque converter clutch would disengage for a moment. A bent driveshaft vibrates continuously, a binding U-joint could maybe just vibrate under load. If it's in the front end, you need to find what's loose. Could be wheel bearings or worn lower control arms or ball joints.