I hope someone can help me here in confirming my thoughts.
I have just picked up my 02 wrx estate and noticed a rather violent vibration coming from the rear passenger wheel area. It gets progressively worse with speed so much so that at approx 90mph on my private road your honour it will cause the entire car to shake.
My thoughts were originally with wheel balance however I then began to notice that under braking at low speeds with constant light pressure on the brakes the car would rock (if thats the right description) to a stop as if the brakes were catching more at one point than the other. So I'm now thinking that the disks may be warped, is there any way of being sure? or is there a well known cause for behaviour like this?

I have posted this in the technical section too but I am somewhat impatient and I'm going to need to know if I'm gonna take the car back to the garage.

 

have you checked wheel bearings? Mine went on the car when i was coming back from wales. Jack the car up and grip the wheel and see if there is any play....the other thing to check are that there are no calipers sticking anywhere as this may cause the brakes to be on all the time....mine were and at about 60 the car went into ultra vibrate mode. when i looked at the disk it was glowing red hot...might be worth looking at that

 

Buckled wheels do similar for reference, not so much a sensation through the brake pedal though.

 

My car did similar when the back discs / pads were replaced. Gentle application of the handbrake as the car was coming to a standstill sorted it. I presume it was debris of some sort. Worth getting the garage to check it if that doesn't work, especially as you've just picked it up. Best of luck. Bob