I've got an Impala 2000 6 cilinders 3.8, and it is having some steering rack problems. The steering wheel stays really hard when the car is stopped, but as I start driving the steering comes back to normal functioning again. If I then stop the car again, I begin having the same problem. Is there any kind of adjust I need to make?

When starting the car and right away and turning the steering wheel it feels stiff, and when getting up to speed too fast the steering wheel starts to shake violently. Sound is kinda like a vibrating grinding noise but goes away when turning a good ways to the right.