I need to ask a question, are you talking about a 1st gen or 2nd gen? '83 up was mentioned but your signature says you drive a 2nd gen.

Here's my experience with a 1st gen. When I changed to ITA I switched out my final ration to a 4.88. Doing the math this means my driveshaft is spinning around 9,000 RPM in 5th at the fastest track I run. I had HUGE vibration problems. I tried a couple different used shafts. Indexing one of them made the vibration tolerable until I can afford a new one. I talked to numerous people that I consider an expert on Mazda's. They uniformly recommended only using a driveshaft from Mazda. YMMV.