Car is all over the road, at Highway speeds I have to really pay attention to what I am doing. Especially if passing, car feels like it wants to fishtail, so easy to loose control. whats the proble?

Have you inspected the tie rod ends?

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Yes sir, everything is tight checked for movement in the wheels while they were in the air. Did notice today that if I make a turn and let go of the steering wheel, car will not straighten, just keeps on turning. I also did a rear suspension conversion a few months ago. At bags went and replaced with a coil spring kit. Not sure if that matters.

Did you find an answer? I have the same problem. Everything looks fine underneath, I replaced tires, had an algnment. No change.

Check your steering shaft, it connects to steering gearbox, had to do mine. I hear it a thing with these types of car.

I loved my 2003 Ultimate Edition Grand Marquis 8 cylinder but it was a nightmare steering wise and a lot of the comments about paying attention while driving if I let go of the wheel just for 2 seconds I would have been three lanes over. 2 months ago I slammed into a house because it would not stop North turn and I could not get the car to go into neutral or reverse to keep from continuing going into a house now I have to go to court even though I was tested for drugs and alcohol and nothing was in my system but it has caused and living nightmare plus I had $4,000 still on the loan anybody have any suggestions I'm from Indiana sucks!

Big rear drive Fords have a shorter caster angle on the front suspension than you might be used to or like. They feel less "on center" than other similarly laid out frames. They are basically a bit whippy compared to a full size rear drive GM product that has a longer caster angle. The shorter caster angle often translates to oversteer in real life road maneuvers. I have found them to be more prone to spinouts in testing. I also dislike the Ackerman angle on big Fords. IMO, Ford never brought their full size rear drive sedan steering out of the 1950s when tight turns were slow operations on residential streets or in parking lots. If you wanted more performance oriented handling, you selected a different vehicle.

I have a 2001 Mercury Grand Marquis and it had the same problem, all over the road and it became extremely scary on the highway. I could not drive it safely after 55 mph. After a lot of trips to mechanics replacing the whole front end suspension included new front and back. Still dangerous, changed the A arms also no luck. Added cop sway bars front and back. That's when I found out what had happened. Ford in there wisdom decided not to add a sway bar on the back of the crown vic or the mercury grand marquis from 2001 on. The vehicle is fine while you have active air ride that works but once it becomes compromised watch out. I almost scrapped the car it was so bad. Also the car had to have the front ball joints replaced also. Once these start to loosen up or seize they cause the exact same issue. I had replaced the ball joints 3 years prior so I was convinced that this could not be the culprit. It was. I met a guy that serviced cop cars and he straight up told me it was the ball joints I paid him to do it and bang drives like new. I had no noise no squeaking so I was convinced they where fine. All this took 2 years to figure out and the car sat foe 3 years as it was so bad t drive.

It's most definitely the ball joints