I was driving down the interstate and then all of a sudden the vehicle just started shaking violently, and I kept hearing thud over and over. At first I thought maybe the driveshaft had dropped. The driveshaft didn't drop. I checked everything else I could think of. Even the two u-joints that are located on the front axles (I guess the axle u-joints). The front Drive shaft appeared to be fine also. I have looked this thing over and over, and cannot find anything. Any ideas?