If it only does when the car is turning you might expect it to be the Diff, but I think I would check the suspension, it could be allowing the axle to bounce up and down somehow. Isn't there some sort of restraint mechanism in the middle which is supposed to prevent this.
Have you tried replacing the universal joints? I had a simialar problem and EB Spares suggested this. I certainly cured the problem. I bought the UJ's and took them along with the prop to a local motor factors/engineers. They changed them for me for about a tenner.
Don't forget to mark the prop at the flanges so you bolt it back together the same way or it will go out of balance. Mine did this anyway and I had to get the prop balanced. That cost £83 and the balance way better and smoother than it had ever been.