It all depends on where/how you feel/hear the vibration.
If you feel the vibration in the steering wheel, the problem will most likely be in the front end. If you feel it through the seat the most, then do a careful inspection on the rear. If it is more noise than "feel", I would have a very careful look at the drivelines.
any "play" in the drivelines can cause vibrations. output shaft, u-joints, pinion shaft. Also, how much axle wrap are you getting or what are your pinion angles like? An unbalanced tire will also "pulse" like you describe, and generally starting at that speed. But you will feel that through the steering wheel or the seat(front/ rear).
