i think i need to go back and delete both of my posts above.
the repair info in general is correct,
it just does not apply to your problem, picture.
i do not know what is pictured,
some part of the trans.
it is located on the main trans body,
not the front diff body.
so it is not the speed sensor or the diff dip stick location.
but it does appear to be a leak point.
it probably should have plastic plug or a cap on it.
and it is possible that higher rpm of lower gears may have played a part.
(but just because you were in 2 or 3 does not mean you were rev-ing the engine.)
or it could just be old age.
i lost the rear output shaft seal on my 97 lego auto trans (w/160k) on the hiway at 70mph.
i felt a ''bang'' under my seat and then it drove fine.
i got to the gas station and as i was filling up,
a puddle of trans fluid ran out from under the car.
i never found a cause, but the seal blew out of the rear.
high speed, high rpm, high fluid pressure, old tired plastic, pop, oops.?????????
you need to look at other auto trans to see what is there .
and change your trans fluid .
clean fluid is always better.
do a search for ''drain and fill three times'' or just ''drain and fill''.
you may need a dealer or a trans shop to help with the plug.
or you may be able to ID it here :
and order it from some where.
the local dealer will cost a little more, 25 - 30%,
but this is a $10 part.
if they have it in stock, get it, it's quicker.
let us know.