My 01 Town Car did the same thing @ 60 MPH.

Motor mount should be easy to diagnose. Lift hood, apply gas and brake at same time. Do not let the car roll. If the front of the engine comes up excessive, the motor mount is shot.

I have noticed it is worse on mine when the shade is all the way back.

The sunroof has two stops. Close the sunroof hit the switch to open, when it stops hit the switch to open again you will see that the sunroof will move back farther. When the sunroof is stopped at the second stop is when you will get the noise. Note it will go away at highway speeds but will make a shuddering noise at lower speeds.