There are several places that a differential can leak from.... If it from
the rear cover... check the bolts for torque. If they are tight, there is
litle chance of the leak getting worse in a short space of time. If the leak
is from the pinion seal, it *may* be an indication of bearing concerns.. or
it may be an axle seal..... Seal failures are difficult to gauge.... The
leak may remain as a minor incovenience or it could get very bad, very
quick. There is just no way to predict with any accuracy. In any of these
cases, it is wise to checked the differential case breather for obstruction.

At the same time, I feel it worth mentioning for all the lay people reading
this.... Foon has fallen into that all too common consumer trap.... Every
summer..... every Thanksgiving and every Christmas, shops all over North
America get the pre-holiday "treat"....

"Hello, Joes Garage...".

"You gotta help me.... you just gotta help me!!!!! I leave for <insert name
of distant destination here> this afternoon and my car is leaking/making
noise/not running right".

"How long has it been doing this?".

"Ohhh, about 6 months or so.....".

And of course, I'm an a.s because I have a family and the exact same holiday
but my car runs (I'm anal about my machines... they may not be clean, but
they are mechanically "perfect"). I even had one frustrated traveller
threaten to sue me because all I would do at 7PM on Xmas Eve was drive him
to the bus depot.

Sorry for the digression.... I just felt the need to rub that in everyones
face....

Anyway.... 150 miles isn't a vast distance.... Without the opportunity to
have repairs made, I would at least ensure that the differential oil level
is correct before embarking on the trip... have the area inspected to be
sure that there isn't a lot of oil clinging to the under-carriage - a sign
that the leak may be worse than we suspect. And, have the breather inspected
for obstruction since a plugged breather will exacerbate the situation.
About midpoint in the drive, get down on hand and knee to check for any
signs that the leak is getting worse - again once you reach your destination
and again midpoint on the return trip. There is no sense in exchanging a
$100 or so seal repair for a $2000 diff repair.

My way might sound anal to some, but I refuse to subject myself to the
frustration of a breakdown.....

Since the OP didn't say if it was the front or rear axle, I'll guess it's
the front, since both of my old '98 and the present '02 both leaked from the
front pinion seal. Have the dealer fix it....I wouldn't want anybody else
reapplying the correct torque to that nut on the pinion gear. If it's to
tight or too loose, you will end up replacing just about everything in that
axle housing, except the 2 shafts.
wth

Front or rear differential?  Does the leak stop after it's warmed up and
driven, or randomly?