Been there...
Okay- let's try to pinpoint the problem..
1. Is the shaking in your seat or the floorboards?
2. Does it come and go, or is it constant?
3. What size wheels are you running?
4. If you have new wheels, do they requre hub centric rings, and do you have them installed?
5. Do you have aftermarket suspension? What type? Is it installed correctly?
I went through the exact same issue as you, about three weeks ago. After numerous attemps to balance my new wheels, adding hub centric rings, and dyno runs, I determined that my wheels actually had a warp in them. Since, I have taken off my 18's and replaced them with the stock set up. Now- no problems.
Here is what you need to do: 1. Get your wheels "ROAD FORCE BALANCED". This will determine whether or not your wheels are true to begin with- you could have dented, bent or warped wheels. 2. Run your car on a dyno machine at your closest VW or Audi tuner.
Make sure that your car is also aligned correctly- get an alignment on a "HUNTER RACK". The hunter rack is designed for VW and Audi, and has settings specifically for Passats. You should be able to find a hunter rack at any VW dealership, or by calling various shops.
If it turns out not to be a wheel issue, it is a suspension issue. This means that you could have problems with either your control arms, or tie rods. This is not only dangerous, but needs addressing soon. Driving with a car that has suspension problems can be really bad for the drivetrain, down the road.
Not trying to scare you, but you should just have a good, reliable mechanic completely check the car over. It could simply be your wheel nuts- make sure they are all tightend and flush mounted.
I know how frustrating this is, trust me. I finally figured out my issue for my car, and since then, the car has never run better. I have KW Variant 1 coilovers, and my fear was that they were installed incorrectly- but I found out that my wheels were the issue. Mine did the exact same thing- at the same speeds as yours.
Good luck and keep us posted.
Best,
psstwgn