Vibration problem, alignment & balance not helping

[kS_Method]

Hey everyone,
I've been having a annoying vibration issue and I'm having trouble figuring it out.
I purchased an 08 S line about 6 months ago, my first all wheel drive vehicle. At around 65+ I can start to feel the vibration and it gets worse the higher I go. It seems to me like it's coming from the rear. Previous owner replaced the front tires but not the rears and I have heard that's a big nono. So I had the car checked out but the drivetrain seems to be perfectly fine. I grabbed a pair of 18s with nankang as-1s (215/40/18 89H XL) to see if 4 matching tires would help but after having them balanced it has toned it down but by only a little bit. I'm still feeling a vibration at 75 mph and up.
With tax season around the corner my next idea was to try getting a new set of wheels and tires. My big question is do I have to get wheels and tires that meet the same specifications as what the car came in (235/45/17 94H)? Or is size, load and speed rating not part of the issue

Any input is greatly appreciated as awd systems are very new to me. Thanks in advance and Happy New Year!

[allstock]

Chasing down vibrations on these cars is a bitch and draining on the resources. Damage to suspension and drive train on these cars has a spreading effect. A lot of times it can be a combination of components even to the axles.

[JayDog747]

How many miles are on the car?

[kS_Method]

Just hit 100k last month

[JayDog747]

Is everything still stock? Control arms, rear diff mount, shocks.

[robsilvr85]

Also, if your wheels are aftermarket, check to see if it is the right hub bore which should be 57.1

[kS_Method]

Yes Jaydog, completely stock.
I have fat fives on at the moment rob and are 57.1 but I'll make sure to keep that in mind thank you

[JayDog747]

At 100k I would change the upper control arm bushings front and rear and the rear diff mount. In my experience, I have found stock shocks to only last 50-60k. You may want to consider those as a point of vibration also.

[kS_Method]

Thanks for the info JayDog I appreciate it, I'm gonna look into this because I think my mechanic said something about cracked bushings in front
So wheel size and all that doesn't matter for all wheel drive though JayDog?

[JayDog747]

Thanks for the info JayDog I appreciate it, I'm gonna look into this because I think my mechanic said something about cracked bushings in front
So wheel size and all that doesn't matter for all wheel drive though JayDog?

DING DING DING We have a winner! The size of the wheel doesn't matter but as robsilvr stated the wheel hub bore can cause vibrations if not properly sized

[kS_Method]

sweet because I like the way 18s look lol
thanks for the help man