Why is my car shaking once it reaches 45-50MPH?!?

I have a 1999 Plymouth Breeze (automatic) and have noticed once I reach 45-50MPH the car begins to shake and/or grind, especially the steering wheel making it hard to handle the car while driving. Also, this is not a minor shaking or grinding this is a "Im-going-to-lose-control-of-my-car" kind. We have replaced the 2 front tires and the 2 rear tires are in good condition, but may need looked at professionally. (My husband worked at a Tire place and has a lot of experience he is saying the 2 rear tires are not causing the shaking.) I have also had brakes, rotars, brake pads, etc. replaced about 2-4 months ago. I dont know what else it could be, the cars in pretty good driveable shape other than this! It will also shake sometimes no matter what speed. Any help or ideas is greatly appreciated!