Shuddering is probably either the pump or the steering rack. Neither will be cheap, parts or labour! If it's making a whining noise then it may well be the bearings in the pump. You can get it reconditioned or replaced. Best bet is taking it to a garage and speaking to them. You can keep on driving it, but you may lose power steering at some point. Not the end of the world, just a bit harder to turn the steering wheel.