I had a similar problem on my 92 Legacy. I had the car into the dealer for brake work, and he told me the left outer boot was very close to failure, so I got it fixed. They checked everything else and pronounced it fine, and off I went. I drove from Ottawa to Toronto that weekend, and on the way home after I got off the highway I began to get this horrible stench through the heater. As it turns out, my right outer boot had failed somewhere on the highway, and dumped its grease onto the cat, which was the cause of the stink. I took it back to the dealer and convinced them (it involved a lot of grumbling) to replace the boot with parts cost only to me. Unfortunately, running at 130km/h for 4 hours with no grease severely shortened the life of the joint, and now I've got the rumbling and vibrating, both through the steering wheel and the chassis when under acceleration (torqueing the joint). So, I guess soon it'll be going back.