Vibration

[AZ986S]

So ever since I acquired my 2002 Boxster S last year, I've been battling vibration in the steering wheel and what seemed to be coming from the rear of the car (entire chassis vibrating), and it is annoying the heck out of me! On the street, I use the OEM 18 in wheels with Michelin PSS. I also have a slightly more aggressive track alignment with -1.0 Front Camber, -1.8 Rear Camber and 0.15 deg Toe in Front, 0.25 deg Toe in Rear. There is no vibration or steering jerking when you touch the brake pedal.

Things I have done so far (partial list):

* Replaced the transmission mounts. Eliminated much of rear vibration from the chassis.
* Had all 4 wheels inspected, repaired 2 rear wheels that were slightly bent (0.035 and 0.060).
* Had one of the rear wheels gouge welded in and refinished. Looks like new.
* Had all 4 wheels Hunter roadforce balanced twice. This last time, the fronts did not even need weights, the rears were much better after repair. Vibration at highway speed is much reduced, but STILL not completely eliminated. Annoying!!!!
* Rear shocks were replaced when IMS/RMS and clutch replaced.
* Front shocks appear good.
* Control arms and bushings look good. Have not replaced any of them.
* New motor mount.
* Upgraded to Eibach front and rear swaybars.
* Added ECS tuning hubcentric wheels spacers.
* Added Rennline wheel studs.

4 things I can think of that could still be contributing to the vibration:

1. Alignment?
2. Older tires? (but this should be corrected during roadforce balancing)
3. Suspension bushings? Seems like they look good (no steering jerk while braking). Maybe it is only the rears then?
4. Axle shafts out of balancing or slightly bent?


Any time I go over 90 or so, vibration/shaking becomes much more pronounced, and I also get this weird buz noise? What the hell is that?

Thanks for any input or ideas you may have.

************EDIT**************

Car has 79K miles.
Replaced all 4 brake rotors and pads 1,000 miles ago.
Tires are worn and yes I should get new ones, but would they not balance out then?