My b/f & I checked out the axle code on the doors of both our vehicles before we bought them. He didn't want anything other than 3.73 or 4.10 in his F350, and I wanted to be sure I got 3.73 L/S in my Exploder.
That's why I love these truck sites. We usually end up knowing more about the vehicle than the salesmen! The salesman I bought my Exploder from had no clue about the gear ratio, L/S.... they can be quite useless sometimes. I can read the tag on the windshield just as well as they can!
That sucks though.... I would complain to the dealer. Just say that you had the diff fluid changed and as a curiosity the mechanic pointed out you had 3.73's, not 4.10's. You did buy that truck over another one because of the gearing, RIGHT?