If it does it in gear but at idle, then it's probably the transmission
itself.  Either way, if you look locally or on dsmtrader.com you should
be able to find whatever you need for a good price.  The cars are now
quite old so lots of used parts available.  If the engine is good and
the body is fine, I'd certainly fix what needs fixing.

And yes, different size tires can really mess things up.  It probably is
the trans itself.  Might be a bad center differential.  You can take the
end case off the trans through the wheelwell and pull the center diff
out with the rest of the trans still in the car.  Probably easier to
just swap the whole trans, though.

And then get a catback and K&N for it and turn the boost up to 15psi and
be amazed at how fast the car then is.