Even the current Shelby has a live rear axle, I think. Afaik, the only Mustang to have IRS (independent rear suspension) was the 2003-04 Mustang Cobra SVT, which was the last of the older shape (SN95) Mustangs. It had a supercharged engine putting out 390 bhp, which must have been some car. Apparently the US market preferred the older live axle set-up as it was easier to modify.
Julian (ex-Mustang owner).