Since you have the IRS from the S550 Mustang it should be easy enough to tell them that and your trans type to get you set up. Not the cheapest option, but a good one. Otherwise, I'll ask my dad what shop he used for our cars driveshafts, I can't remember off the top of my head.
Thanks for pointing this out, I just went deep in a rabbit hole and solved a paranoid drive shaft vibration issue that has been plaguing my mind for a long time.