Also, it should happen, and even more, when at a standstill, which puts more stress on the steering system than when moving; yet you said it never clunks at a stop.
So, I'm thinking it may be about how the torque from the drivetrain splits left/right along the axle. When going straight the split is even 50/50, but as soon as soon as you turn, the split shifts in order to let one wheel turn faster/slower than the other. That's the spider gears' job, and they are the mechanism between the ring and pinion and the axle shafts, inside the axle differential (the pumpkin).
I'd say that there's a some play or slop in there, and that causes the single clunk.
Try this: have someone drive your vehicle slowly while you walk beside it. First straight, then ask them to turn, and listen for that clunk. Try walking to the left side of the car and then to the right, try and hear if it seems to come from one wheel or the other, or if it's from the middle... if it's middle: bingo!
Another cool thing I've seen a UAE Jeeper do is attach his Go Pro under the car, and record not only the video but also the audio. Where his clunk was coming from was real obvious from the video clip. So, if you have a Go Pro and clamp mount, that's another cool way to use it 8)
Gluck with your clunk, and please let us know when you resolve it
