Under certain conditions, mostly taking off slightly fast, I feel a slight shutter. It doesn't feel like it's coming from the trans, if definitely feels like it's coming from the rear end. This is my first Mustang, am I feeling a normal clutch pack bite of this particular locking differential or is this a problem?

 

The only input I have would be maybe wheel hop that occurs with our cars. I am unsure exactly what you mean by shutter perhaps. Is traction control enabled?

 

It's almost like a slip, but very mild. If I wasn't the driver I would never notice. I always drive with traction control off. It's not wheel hop, it's taking off slightly fast, or should I say one notch above slow. Nothing that would make the tires slip.

 

It could very well be the clutch pack. The ones that came in most of the cars were defective anyway. After a little while they start making this horrible squealing sound when you make slow speed turns (like driving in a parking lot). There are TSBs out for it.

...but I have never heard of,or experienced,a "shudder" like you describe, so it's anyone's guess.