I was gonna say the same thing, I got a air bubble under a steel belt and it shook like the drive line.
I had a friend driving behind me and said he could see the tire come up off the road because it was egg shaped.
This is not a four wheel drive is it??? We just pulled a front drive line on a buddies truck because of his universal joint.
It could also be a wheel bearing going out. Probably on the back since your not feeling anything in the steering wheel.

Kurt