HI, I have 89 mazda rx-7 tII. And I have had numerous problems, but this one is kinda frustrating.

I put some used tires on the car and had them balanced, and the car was driving fine. However a week and a half later the car began to shake from the rear really badly each time i would accelerate to about 40-60 mph. I then had brought the tires in to get re balanced...but upon checking them they were all in balance just fine. Any ideas? thanks!

 

Inspect your drive shaft, samething happend to my fathers old 7. The U-joints are not replaceable, If your drive shaft is bad the whole driveshaft has to be replaced. Along with rear trans seal and fresh trans fluid. Could be something else tho of course. Wheel bearings?