The IRS in these cars is the biggest love hate component. I swapped one into my GT with complete FTBR bushing kit and toe links 10 years ago and you couldn't pay me to go back.I ran an 04 irs in my Mach for 5yrs. Those were the unhappiest years. Couldn't stand the clunk, the highway vibrations, the floppiness. The 100+ extra pounds sucked too. Poly diff mounts and aluminum driveshaft reduced driveline vibrations some, but they were never completely fixed. It turned a little better with the irs, took bumps isolated to one side of the car a little better, and the exhaust was cool to look at from behind, but at the cost of everything else it still wasn't worth it to me. I switched back to sra, built it, and would never go irs again. If I was looking for a Cobra, it would have to be totally stock, down to the air box, and 1 owner, or I wouldn't buy it. Which eliminates the vast majority anyway. But then I'd sra swap it first thing. Even more than foolishly installing stock brake pads, that irs was the worst thing I ever put in my car. 100%.
Probably not the only mod eitherI think a lot of people consider the IRS a sign it was flogged repeatedly at the drag strip
Probably not the only mod either
Other possibility is that it was swapped early in the cars life by someone who wanted 'reliability' and was against IRS, and didn't want to build one etc