[wtm442]

Jack up the rear end (add jack stands) until the rear tires are off the ground. Put chocks on front tires. Now run the engine RPM's up slowly while its in fourth gear. Does it still shake only at 30 MPH? What about 60 MPH?

Now turn it off and start looking at driveshaft angle. Rotate tire slowly by hand to see if one of the rear tires is way out of round (did you have the tires spin balanced?). Is one of the rims bent?

If there is no shake with rear end up in air, maybe its in the front end.

I had a defective tire one time that was so bad the car wobbled at low speeds of 15 to 20 MPH, but was hardly noticeable at high speeds. It was a tread shift ... noticeable when you rotated the tire. This was about 6 months after a balancing and front end alignment.