According to statistics published by the IDF, the city with the highest rate of young people serving in the IDF (i.e 18-21 year olds and not career soldiers) belongs to Modiin. At the bottom of the national list - Jerusalem (52nd) and Tel Aviv (62nd).
Jerusalem is fairly easy to explain - large Haredi and Arab population. But what is the explanation for Tel Aviv being at the bottom? Originally I thought that perhaps Bnei Brak was included inside Tel Aviv's status - however in the list you can see that the Haredi towns (Elad, Bnei Brak, Beitar Eilit) are all counted individually. Interestingly enough, when Ynet tried to work out the statistics according to regions - they removed Bnei Brak from the "Mercaz" region - to make it look a little better. This might actually be the dream of many people in Gush Dan, including those living in Bnei-Brak, but it is little more than cheating on the statistics.
I'll also note, that while Ynet, Ma'ariv, Israel Hayom all noted the low rankings of Tel Aviv (though without giving an explanation), in Ha'aretz this fact goes unmentioned. The piece in Ha'aretz actually deals with the ongoing debate about women singing in the IDF, but it is the only mention I could find of the statistics on their webpage.