I can confirm 100% replacing the 2 rear CV driveshafts fixed the vibration issue. I no longer have speed specific vibrations on the highway! Best of luck to all others with vibration issues - I hope this information is useful in your resolution.
It is difficult to isolate the problem to the rear driveshafts as the car goes into limp mode with the rear shafts removed due to a speed sensor linked to the rear diff. My dealership bit the bullet, took the chance, and replaced the rear CV driveshafts as a last ditch effort and it paid off.
Just wanted to provide an update to this as I was able to drive the car about 300 kms after the dealer replaced both rear cv's. Before I get to the good stuff, I want to personally thank cr8trface for documenting his encounters-- without it, we wouldn't have the info needed to resolve what is an obvious problem Mazda wouldn't acknowledge.
Read the entire thread for the full info. A quick synopsis.... after experiencing significant vibration whereby the front visors and front seat would visibly shake I thought it may have been unbalanced wheels. Balanced them at a shop using the typical balancing machines. No dice--still shook like a leaf.
Did some research and brought it to the attention of the dealer while under warranty (less than 20k kms, also purchased new directly from Mazda).
The service manager attributed the vibrations to just how Mazda develops and makes their cars, you know zoom zoom... Stiff suspension, low profile tires--developed to enhance road feel.
I simply didn't accept these reasons and offered a different explanation-- maybe something is off.
With no further help or explanation from the service manager at the Mazda dealership, I took the vehicle to a shop with a hunter road force balancer on my summer rims and tires (the falkens). They balanced it to acceptable spec and it was marginally better but still no dice. This told me it's not the rims or tires.
Again, the service manager at the Mazda dealer insisted it's probably the low profile stock wheels and said time and time again in a condescending way the car drives normally, which I didn't accept because I have NEVER had this experience in a vehicle before--let alone in a flagship.
I sent the service manager an email and copied this thread here with cr8trface's detailed comments. He then stated he thinks the vehicle drives normally and again offered the zoom zoom explanation.
He then reluctantly said he is willing to change the rear CV shafts to pacify my continued persistence but promptly mentioned after this, NOTHING ELSE.
I spent the next few weeks deliberating if I should do the rear CV replacement--under warranty at that. You see, I know very well 'if it's not broke, don't fix it!' I didn't want the vibration to worsen, if at all possible. You just never know.
I decided to wait until the winter season and pay out of pocket to do the road force balancing on my winter set rims on x-ice tires. It passed and was within spec. It was neither better or worse. I then decided i don't really have anything to lose and I couldn't imagine the vibrations being worse than it already was...
The dealer changed both rear CV shafts and I can confirm, just as Cr8trface mentioned, that was 100% the problem. The car drives so so so smooth it's ridiculous that I've had it for this long, with my now 6 year old daughter in the back vomiting on car rides because of the strong vibrations. She sits in her car seat and excited to go on drives now without issue. No more vomit bags in tow.
At all speeds the vehicle feels really nice and smooth. The seats and visors are both steady as it should have been from day 1.
What gripes me is from day 1 I was lead to believe this is just how the car behaves, you know, zoom zoom. I had to spend money out my own pocket to get to the bottom of it, which really put a damper on family trips/ drives.
If like me, you know the car didn't hit a pot hole to cause damage and you've ruled out the balancing of the rims/ tires, I strongly suggest you ask the dealer to look or change the the cv's-- maybe an installation problem at the factory?
Hopefully Mazda can step up to the plate and fix this for customers because I've seen enough reports on it to think it very well may exist on the cx-5 & cx-9's.
Goodluck all! Let me know how your approach goes and how your service manager attempts to explain his version of zoom zoom.