I'm pretty sure that wasn't an option in '06, all GT's had 3.55 Limited Slips. Not 100%, more like 80% sure of it.

 

Mark - according to the 2006 Mustang order guide on the page numbered 4 in the guide, the V6 cars and the automatic GT's have a 3.31 rear end and that for the GT, the rear end is limited-slip.

Here's a link to the file if you'd like to read it: http://bradbarnett.net/mustangs/timeline/05/06/2006_Mustang_Order_Guide2.pdf

 

flieger67, roger that...I guess the brochure had a typo in it...thanks!