4WD

Was driving on the highway in 2WD when it
started smelling like eggs and making a
clicking noise from the center of the car.
Shop said the gears are stripped and that I
blew the front end. I’ve only owned the car
for 6 days and out of those 6 days the car
has been in the shop three times. After the
clicking noise I sent it to the shop for the
4th time. I know like almost nothing about
cars at all. Can anyone tell me if this might
have been a pre-existing problem or if this
might have been something I did wrong.
My bf showed me how to throw it in 4WD
earlier the day before, the owner of the
shop told me I was lying when I said I was
driving in 2WD. Background info: they just
changed the universal joint early that day, I
was told there was NOTHING wrong with
the car when I bought it, on the 28th we
bought the car didn’t pick it up until the 1st
cause it needed to have its rotting break
line replaced and it needed a new valve
cover. Next day the check engine light
came on, needed a tune up and it had an
engine missfire. They fixed that on the
third. They took my drive shaft out cause
the universal joint was bad(something that
obviously was bad when I bought the car
even though I was told NOTHING was
wrong with it) had it replaced the morning
before it went back into the shop for this
incident.