Hi Devildog and all
I know it has been over 3 years since this post was first initiated but I have exactly the same problem now with my 2012 automatic transmission mustang. I went to to the dealer since the car is still under warranty and they changed worn bushings in the differential but the sound is still there as I come to stop and although it does not rock the car in a way to move it but you can feel a push and shake from rear side.
Any input would be much appreciated. Thanks!