Long-time lurker in this thread as I was dealing with the same vibration issue as nearly everyone else.

Earlier this year I got a mix of the old and new version of K-power's kit for the ZF6: new gray frame rail braces and cross member, new steel adapter, old steel driveshaft (from Indy Driveshafts), old black differential mount.

My first driveshaft adapter needed to be sent back because they didn't chamfer the trans-side of the hole for the output shaft, the second one bolted up fine.

After getting the car back together I identified a vibration that started when the driveshaft was about 5300RPM (roughly 100mph) and which got worse with more speed. I identified a significant amount of runout in both the adapter and driveshaft.
K-power wasn't too helpful, offering to either rebalance the shaft or give me a $300 credit on their new $1100 shaft I didn't feel like jumping through hoops for months to get nowhere, so I ended up taking a stab at a 2-pc driveshaft setup.

Had an '01 Z3 3.0 auto driveshaft modified to mate up to the miata differential, put in a new E36 M3 guibo and CSB, and had a local guy fab a bracket for the CSB and modify my exhaust to fit with the new drivetrain.
Completely fixed my vibrations, its been to about 130mph on the road and in the air (driveshaft at ~7000RPM) and it's like butter compared to K-power's shaft which felt like the car would shake apart at 115mph.

 

It's always been the shaft. Mine was an early kit and may have been a different driveshaft shop. Or I did a great job of centering, which I did.

 

Nice! Glad to see the 2 piece shaft worked for you. Wish I'd seen someone do that when I was diagnosing my vibrations.

 

I've been following these threads for years. I was an early adopter of the ZF6 Kmiata kit (purchased in 2020, or maybe 2021 I believe) and have always suffered from a vibration which made the car very unpleasant to drive. It's sat in the garage for around 2 years because I simply don't want to drive it! It was hands down the worst thing I ever did to the car, and I got rid of the PPF, rear diff and sold the gearbox, so a complete pain the **** to revert to OEM. I'd frankly had enough after swapping in 3 different shafts, alignment blah blah. I just lost interest in the car and it's sat there for 3+ years in my garage.

Well, finally, I think I'll try this 2-piece setup inspired by Zymos and Greg Peters. I just bought a propshaft on eBay; from a 3.0 (m54) e85 Z4.

I'll have a centre support bar fabricated and the rear end of the shaft modified to fit my Getrag rear end.

Crossing all my fingers and toes this works. If I could finally fix this, it would be a game-changer.