Slight Vibration in the rear
Springs
Bad Pavement
Bearings
Gears
Crooked license plate
Drive Shaft
Caked on snow or mud
Loose/broken suspension components
Loose wing/spoiler
Earthquake
Wheel out of balance
Deformed tire
Gremlins
More detail from the OP would be helpful, but a "vibration" could be a lot of stuff. A license plate would probably not make something one could feel in the butt, but vibration noises could be made by a loose license plate, loose exhaust, loose jacks/spare in the wheel wells, etc.
If we have more info on what the condition is and how/when it can be recreated, we should be able to shorten the list.







It interrupts laminar air flow over the rear end, creating disruptive vorticies, resulting in a air-hammering effect.