Subaru - Forester :: 2000 Started Shaking At Higher Speeds

My 2000 Forester recently started shaking while driving at highway speeds or while under engine load (hills, accelerating, etc). My first thought was that it was a problem with the alignment so I took it to my local tire shop. The car has 164K on it and has been without a Subaru specific mechanic since my last guy got fired a year ago, though I am trying to get an appointment with the only other Subaru mechanic in the area, outside of the dealership.

The tire shop found an issue with cupped tires (naturally they'd want to sell me tires) and told me that my passenger side rear CV joint was bad and they'd have to replace the axle. Since this sounded credible for the symptoms it was showing and they found me an axle, I told them to go ahead and replace it.

When I got it back the first time, I didn't see any great improvement in the shaking. I also noticed that they'd forgotten to put the bolt back into the bottom of my shock absorber. o_O When I brought it back to them, they fixed the missing bolt and agreed to keep it for a week to troubleshoot where else the shaking might be coming from. In the process, they replaced spark plugs and wires (which it probably needed anyway) and did an oil change, but the shaking is still there after all their tinkering. They suggested the problem may be in the timing and that I should have that checked out.

I don't think they're right about that. The vibration feels (to me) even more like a CV joint problem now than it did before. It shakes more violently and at slower speeds the longer it runs so by the time I get to work (40+ miles) it's almost constant. It stops shaking when you pop it into neutral and let it coast suggesting it's not timing or alignment - and making me think even more that it's something to do with the drive train.