Your TC is directly under its shifter (2H-4H-N-4L)
There might be an OEM skid plate covering it, I don?t remember.
This;
is the TC, peeking out just in front of the cross-member that holds the T-Bar adjuster.
This;
is the TC with that cross member out.
This is it out of the truck;
You?re front and rear drive shafts are attached to it on ether side. I doubt it is leaking but would still look.
It seldom leaks unless you crack it by smacking it or someone has been in there and did not seal it up properly.
I doubt your mechanic did not change your diff fluid (it is an easy job so a shop still makes good money doing it) and agree that it would not have ?tape? from the factory.
Sounds to me as if this ?oil change? place is trying to rip you off, why didn?t you go to the original mechanic for this?