I have a 96 Grand Marquis I bought over 2 years and nearly 25,000 miles ago. It has 110,000 miles now and i am experiencing a slight shake in the car that I believed was engine related. A friend owns a slightly newer one and says he has the same problem and mechanic told him it was the cracked plastic intake manifold. I now believe it is likely the transmission shuddering. It happens mostly under acceleration and under a bit of a load like climbing a hill.