That's strange since from my understanding these issues are actually what a 1 piece shaft is usually installed in Mustangs to alleviate. The Mustang 2 piece shaft with the carrier bearing has a MPH limitation and it's known to vibrate from failed carrier bearing bushings sometimes even in cars with lower mileage.
I'm not a fan of E85 but to each their own: E85 Conversion | Ford Mustang - LMR.com