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?

 

It could be a combination of the three, wheel balancing, warped discs, wheel alignment, they're all not really that expensive on their own, but together........ hope this helps.

 

Start with one and dismiss them methodically. Have the tracking done and see if this corrects the problem. This would be the most obvious cause and easiest/cheapest to rectify (also have a look at tyre wear). Have the discs/pads checked next if the problem has not been corrected