Anyways I see two problems with this. The first is as many people already said its Locket/Warmogs build that has made those J4s and Xins nigh on unkillable tower divers. If you wanna nerf something nerf those two items. Second thing is the high outbreak of J4s and Xins is because of their two new awesome skins. It isn't entirely due to their being so good in the jungle. They are actually slightly lacking in top lane if anything.
no no no, that is not at all how you do it
if you nerf Locket and Warmogs, you affect every champion in the game. If you nerf Xin Zhao and Jarvan IV, you affect Xin Zhao and Jarvan IV. Also the skins have nothing to do with it, Xin at least was an extremely popular pick for a good long time before this, a big reason being with the new stack health meta he's a machine
http://i45.tinypic.com/jrfwjl.png "The Lakers traded their picks for a 40 year old hobgoblin." - bond21
I can see Xin needing a mild nerf, he's pretty strong right now. But Jarvan? Come on. Sure he has a lot of utility for a bruiser, but he's not the strongest elsewhere. You generally need to itemize him for pure damage or as a tank, he can't 1v1 very well. Either he doesn't have the damage to put someone down, or he's just an ADC with disruption in exchange for range.