To disable the standard limited slip diff - you can't.
If you have deep pockets,
www.traction.eaton.com , and check out their ELocker, an electronically controlled, high perf, locking diff. (You would want to turn this off for normal street driving, to allow free wheeling of the rear tires so you could negotiate tight turns without the tires 'scrubbing' like a dump truck.)
But, I think what you really want is
http://www.ponyexpressperformance.com/tractioncontrol.html , 52.95, including shipping.
The Diablo Predator, $419, (hand held tuner) is the product that can disable the Traction Control via tuning.
Personally, I do like the Traction Control when it's raining, but hate it otherwise. Hate it because I often forget to turn it off. The $52.95 option fits the bill, and is a heck of a lot cheaper, if this is all you need. Also, I'm sure there are other companies out there making a similiar product. I found this one simply by googling for "mustang traction control disable". Best of Luck!