find out where it is coming from. wipe down your tranny clean......and look later. stick cardboard under the car. only a couple places where it would leak. more then likely halfshaft seal. i forget the "proper term". or possible it is leaking from where the shift linkage connects to the trans. (hopefully not since you would need to take the trans apart). the shop has a good argument by saying you had no fluid in the beg....so dont go that route. even if you put a new diff in.....it still would leak. there is a seal somewhere that is broken. (or crack...but highly doubt that)