I'll admit that I have the Mercury variant of this van, but the two are almost identical. What I have noticed is that driving around 35 mph, turning the steering wheel gently back and forth, you get a jerky feel through the steering wheel, but only as the steering wheel is actually turning. You do not feel this once the wheels are at the new direction, only as the steering wheel is moving. You also do not get this feeling at highway speeds, or low speed maneuvering such as parking lots. The problem appeared quickly, the difference of me driving it one week, it being fine, the next week not.
The tires have been rotated / inflated / balanced in an effort to solve the problem, but it persists. The power steering pump was leaking, and was replaced, but the problem started before the power steering pump was replaced. Belts and all of that are fine as well. The van has a little under 25K miles, has the factory rims with Goodyear Eagle LS tires. It has been to two dealers a total of three times, and the problem still exists.
Ford / Mercury is at a loss in their service departments, but since the Quest was Nissans main minivan model, I am hoping someone familiar with that model has more experience with it than Ford appeared to have.