well, an EASY 5 minute answer? nothing will help fuel economy, easily.
but if you want it bad, then yeah....
all comes down to this:
how MANY hundreds of dollars are you willing to spend, to save a dollar per day?
if you get like, 24mpg NOW, and you get a turbocharger for somewhat cheap, used,
for say... 400 dollars, and it gets you up to 27 or even 29mpg, how long will it
take before it SAVES you money?? one thing that would probably affect your mpg
noticeably, good or bad, would be taller or shorter gears. or, if you don't use your
4wd mode, like, EVER, converting it to full fwd mode would help a lot. even if the
transfer case isn't engaged, it still creates a slight drag. parasitic loss. and the rear
axle drive capability adds a lot of weight. like, as much as another grown man.
I think this is most likely your daily driver, if you care about a few extra mpg. so that
rules out my other suggestion. porting and polishing your intake and heads and exh.
it can be done for basically free. it just means taking apart your engine for a week.
and a new set of gaskets to re-assemble. people usually associate a ort and polish
with more performance. not mpgs. but, think about it, anytime you add more power,
without adding more fuel thirst, you always have the option to put a little less foot
on that old pedal. don't look at it as more power at the same rpm. look it at equal power
at less rpm.
a nice exhaust can help a lot with midrange power. and, surprisingly, freeway isn't
high-end rpm. unless you are really out of tune or tired, or tragically underpowered.
most likely, you have enough power that, once the weight gets moving, 55-65mph,
your engine is not working at full throttle, its simply maintaining that already earned
inertia. hence, big heavy V8 trucks, being able to shut down half their cylinders even
going 60mph with 6,000lbs. I bet your legacy doesn't weigh 6,000 lbs.
sure, your legacy doesn't have "variable displacement" capability. but the logic is the
same. once you get going, its not working very hard to KEEP going.
so if you get an exhaust and a cam profile that helps MIDrange, freeway driving is
improved a LOT.