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Just to rule out your wheels and tires, get them road force balanced on a Hunter 9700 series machine. Many Discount Tire shops have them, but some don't so ask. With this machine, they have the ability to see the heaviest part of the wheel and the lightest part of the tire and match them to make the unit more balanced without weights.
If after you've done this the car is still shaking like crazy, a Porsche mechanic (dealership or indy) needs to inspect the car and drive it. He can then tell you if one of your wheel bearings is shot or not, which can cause bad vibration. My car had a horrible vibration problem that I could never fix with wheel balancing. I changed out the wheel bearings all the way around and it rides like a brand new car now.
These are the things that spin and cause vibration at speed. Another thing to check is your motor mounts, but frankly, they degrade over time and don't crap out on you overnight like what you are describing.
If it's a bearing, you should find a good indy shop to do the work for you. The dealership will rape you. The part is only $50 but the book rate for labor is something like 4-6 hours. An indy might just charge you actual time.
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