A couple of weeks ago when I was driving my 2wd Saff the steering did'nt feel right, i have had mine for nearly 24 years and you get used to how it feels. I knew something was wrong and took it in to Richard at Tremona Garage.He soon found the problem was the 2 fixing bolts on a new steering rack I had fitted a while back had somehow come loose. Tightened them up and lock tighted them and the car was back to normal! Not saying this is your problem but its something I would never have thought of.Steering coupler could be worth checking as well.