Does it only do this when the wheels are turned or just when you are over 40. Usually if your turning and going under speeds of 20mph you'll hear your outer joint knock. You'll hear it if you also back up relatively fast. This sounds a lot like a joint issue. Speedway auto parts have some good axles, as do carquest. Our shop tends to frown on most advance and autozone stuff. Honestly i do to.
I would also look under the car if you have a jack and jack stands and see if grease is leaking anywhere. Like near the axles or on the int. shaft.