1995 Sedan Deville rear end shimmy

I have a 1995 Cadillac Sedan Deville with 131,000 miles on it. For a while now I have had a shimmy back and forth that can be felt at any speed. I strongly think it is caused by the rear of the car. I do not feel it in the steering wheel at all or when I apply the brakes, but at slow speeds you can actually see the hood ornament moving side to side. At high speed it feels like a tire with a flat spot on it. A friend of mine rode next to me the other day and said they saw nothing abnormal with the tires. A while back I did have the rear right parking brake stay on which may have warped the rotor, but I feel nothing in the brake pedal ever. What could it be?