2003 Legacy: rough ride, poor gas mileage...

I've got two 2003 Legacys - one is a wagon SE the other a sedan GT. The sedan drives like a dream, is smooth, quiet and gets good gas mileage - I am
quite delighted with the vehicle. The wagon drives very loud and rough, with lots of vibration through the body and especially the shifter and pedals. So far the vehicle has had a couple of repairs including a replaced rear diff (at about 5,000km) and a replaced clutch (at about 35,000km). Ever since replacing the rear diff, the wagon feels like a cheap pick-up truck. It bucks at low rpm in 1st and 2nd gear, it occasionally stalls on take off, and seems to have excessive engine breaking. On top of this, I get horrific gas mileage (about 25% worse that the sedan when driven on the same routes).
Almost from day one with this car (it now has about 35,000km), I have complained at each of my scheduled service appointments (Marino's in Toronto) about these issues. Each time they tell me that there is nothing wrong with the car. I explain that the two Subaru's are like night and day and that there can't be this much variability with the vehicles. It is almost as if there is some sort of friction somewhere in the engine/tranny/etc. that is cause roughness, excessive engine drag and poor gas mileage - but they are at a loss to diagnose the issue. The only thing they have offered is that the next time a Subaru of Canada guy is in town that I bring the car for a test drive. Shouldn't their mechanics be capable of doing this themselves? I am getting a bit frustrated with this service department as their first reaction to any problem seems to be deny that anything is wrong and it always takes several visits before they will actually do anything about problems (this has happened with blown speakers, a sterring wheel that was no where near centred, internal squeaks and rattles, a buzzing from the rear diff, and a noisy and extremely stiff clutch).
In any event, from the symptoms described (rough ride/excessive engine breaking/poor gas mileage/etc...), can anyone suggest what may be wrong with the car and what should be looked at? Any suggestions for next steps to get this resolved??
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Napalm writes :

If I were you I'd try another dealer or two. There must be at least 4 or 5 in the Toronto area.
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<Napalm> wrote:

Is the GT AT or MT?
--
Dominic Richens | snipped-for-privacy@alumni.uottawa.ca
"If you're not *outraged*, you're not paying attention!"
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Both of my vehicles are MT. I once rented an AT wagon (regular L) and found it to pull extremely hard compared to my MT wagon.

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