HELP--Driveshaft auto-brake area throbbing vibration

The throbbing that you are describing sounds like the emergency brakes need adjusting. If you park on a steep hill and put it in park will the brake hold? When mine need adjusting they will throb when rolling down a hill. The other thing that will cause that is a failing green pressure switch, it will leak causing the brakes to drag or throb. Another sign you can look for is the brake drum itself,does it look blue in spots indicating heat caused by dragging brakes.