Unless it bothers the heck out of you or is really loud I wouldn't bother with it. My F-150 has had a harmonic that sets in about 41.5mph and 83mph since day one. 113K and it's still fine.

Your service manager is an idiot, BTW, all the gear sets are adjustable, that's how they are set up to begin with. The problem is that almost none of the service departments have techs that mess with differentials at all. Most of them have never worked on one other than to change out the entire thing (big whoop).