the clunk is coming from the rear-end, every 8.8 rear i have ever owned has made the same clunk, the 2 piece shaft has very little play in it.
lift the car and hold the pinion, the stock DS is very solid
This has been my experience as well. I didn't realize that these Mustangs had 8.8s in them. I have a Superformance Cobra replica that has an 8.8 IRS and it clunks like crazy at low speeds. I don't hear anything during normal driving but the side pipes are pretty loud so I probably wouldn't hear it. I called Ford Racing and they said it was normal. I know there is a lot of play before the driveshaft starts to move when I turn the rear wheels while they are off the ground.
I hear gear slap or clunks all over the place on my '11. I often hear the gears slapping when I let out the clutch and start to move forward. It sounds a lot like letting out the clutch too fast but that is not the case. I have eased the clutch out and it still does it. In fact I am sure the clutch is engaged and it still happens. This is different from what I hear in the rear.