1998 Plymouth Voyager Rear End Vibration

I have a Plymouth Grand Voyager that has developed a substantial rear end vibraton.
At very slow speeds, the vibration is exagerated to a wobble.
At higher speeds, the vibrations seems to diminish. The faster I go, the less noticable it becomes. At 75-80, it is almost unoticable.
I have examined the tires and tire pressures, but nothing has become of those searches.
Last, but not least, the vibration at highway speeds, seems almost rhythmic. The vibration starts out as normal, then over a span of a few seconds it fades away, then fades right back in again. This pattern continues over and over about every 2 seconds or so. I've never experienced anything like it.

Any ideas?