life expectancy of the rack & pinion system is?

Shimmy is probably a tire problem.You could have worn tie rod ends as well.Rare to change a rack at any mileage,though I have heard of it.My is fine in my 2002 Taurus with 175k on it.To keep it alive, a good idea to render some sort of power steering fluid change.

as camster said, rack n pinions dont cause a shimy, when they need replaced its usually cause the seals have went bad and are leaking p/s fluid. the first thing to check is the tires for a broken belt or wheel outa balance. does it shimmy all the time, or when applying the brakes? does the steering wheel vibrate or wiggle when it shimmys?