Thud sound, shattering movement

I have a subaru outback, 2000 model.
I bought the car a few months ago to
drive around in Chile on vacation. The
roads here are terrible, gravel roads
with big stones and potholes. I have
driven approx 800 kilometers on
these gravel roads, and approx 4000
kilometers since I bought the car.
When I bought the car I had a
mechanic who looked at it, he
changed the oil and the filter,
alternator band replaced, rotors and
calipher brackets replaced, timing
belt and air filter also changed. After
driving 800 kilometers on the gravel
roads I noticed a thudding sound
from the car, and also a shattering
movement that comes with the
sound. I feel the shattering in my feet
and my seat. The thudding sound
came when I accelerated, and when I
was driving in more than 80 km/h. I
got a mechanic to look at it, he said
that a piece in the central suppot axel
needed to be replaced. However he
didnt have the piece and sent me
away. I drove to the next town and
got another mechanic to look at it. He
said it was a problem with the front
dampers, but didnt have the pieces
and sent me away. I drove to the next
town to see another mechanic. He
said that the constant velocity joints
needed to be replaced, which he then
did. He also said that the engine oil
level was low, so he refilled the oil. I
got on my way, and after driving
about one hour, the thudding sound
and shattering was back.

The faster I drive, the faster the
thudding sound (sounds almost like a
helicopter), and the harder the
shattering movement. When I brake,
the sound and movement disappears
to a great extent, sometimes
completely. But only for a short while,
and it comes back when I accelerate
at higher speeds. Yesterday I drove
several hours at around 100 km/h,
and it seem to have gotten more
persistent. Before yesterday, the
thudding and shattering came and
went, it could disappear after braking
or going downhill. Today however,
after 10 minutes driving in 80-100
km/h, the thudding and shattering
was back. In low speeds it is less
present, but it is more present in high
speeds and especially when
accelerating, and it is now less
interrupted by braking or going to
lower speeds. Sometimes the
thudding disappers when I turn to a
hard left (the car pulls more to the
right). I also checked the oil level, and
the mechanic has overfilled.
Sometimes I also smell gasoline
when I start the car, not always and
not a very strong smell. I am at a loss
here, the mechanics cant seem to
figure it out. Any input would be
much appreciated.