Almost anything in the front end can cause what you describe. Start by checking a few things with the front end off the ground. Check to be sure your wheel bearings are adjusted correctly, your upper and lower ball joints are ok and that your bushings aren't shot. Any of these can cause a movement in the front that transfers back to the rack and pinion. They are all pretty easy to check, with the wheels on try pushing and pulling the front in all directions. This is easier with the steering locked.