Oh sweet, good to hear. Where at? Also, might want to go look at another dealer to install these if you are not DIY or get a Mustang shop to do it. Any dealer that tells you what they did is trying to scam you out of a LOT of money for a brand new IRS.
You could also have upgraded to 03/04 shafts if you wanted but that's a lot more work and not needed for what the stock motor can take. Biggest killer of the rear axles is wheel hop which is caused by the junk compliant rubber bushings Ford used in the IRS for NVH reduction. There are kits out there to completely remedy the IRS wheel hop issue but they are $700.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.