smiffy6four

Changing the front shafts from side to side can often eliminate vibrations. By swapping them over, the joints drive against the relatively unused (only used when reversing) thrust faces of the joints. I do this to my 99 Forester once a year, just to get longer life from them.

Have you tried adding the AWD fuse? That puts the car into FWD and stops sending power through the rear driveshaft. IF the vibration is still there with the fuse in, then you know to check the front. Sometimes, it's useful to swap the front cv shafts, side to side. If the problem moves to the opposite side, then you have your answer. If it doesn't make any difference, then you may gain more life from the cv's anyway, as they are now driving on the less worn faces of the cv joints.

Another vote for the drive shaft.

Was the Turbo a genuine Subaru replacement? I'm a little confused as to why you think Subaru should offer warranty on work done elsewhere, on a car that's out of warranty.

You'll need a universal joint for a 3/8 ratchet, and a couple of extensions. Plug gap is 0.042". Take the wires off one at a time. I always add a small smear of antisieze compound to each plug thread; this makes them easier to extract next time and prevents carbon from creeping up the threads, which can abrade the threads in the aluminum cylinder head. Take your time, don't overtighten the new plugs.

The yellow font is really hard to read. How about plain old black?

Or just cut a strip of sheet metal, stick it on the inside and bondo over it.

Your mpg sounds about right; I live in similar terrain, drive a 99 2.5 auto Forester and get 25mpg locally, and 30 on a highway run.

The valve in question is to shut off the tank vent and the evap hose; this is required to allow pressure to build in the tank while fuelling so that the gas station nozzle will see the rising pressure, prevent overfilling of the tank and shut off the nozzle. source: Subaru Factory Manual

Ditto

Headgaskets can be changed with the engine in the car. The trick is to remove the lower nut from the motor mounts one side at a time and jack up whichever side you want to get at. Place a block of wood under the mount as a safety precaution to avoid the engine falling.

The stuck piston is easier to remove when it's stil on the car. Block the one that moves and use the brake pedal to create hydraulic pressure. Run the car if needed to have power assist.

Conventional brake fluid is hygroscopic(attracts water from the atmosphere); I use synthetic fluid when doing a complete brake job. All hydralic parts last waaaay longer that way, since synthetic doen't attract water and lubricates better. Just make sure to flush brake lines of old style fluid.

Is one of the rear brakes siezed? Sounds like you need to get the wheels of and have a peek. Do you get the same results when pulling the parking brake at low speed?

Year, model of car, etc. Could be a blocked tank vent; evap has nothing to do with filling. The evap system is closed by a valve actuated by the flap just inside the filler neck when re-fuelling.