Mine doesn't have any nuts on those either, I keep meaning and forgetting to put some on, but I haven't heard any strange noises from there yet.
I believe they are an M6 nut.
2013 JKU Rubicon - Commando
Teraflex 2.5" spring lift, Teraflex monster front and rear trackbars, G2 wheel adapters, Goodyear Wrangler MTR Kevlar 35 x 12.5 x 17, Superchips Flashcal, Warn Zeon 10, Smittybilt XRC bumpers front and rear with tire carrier, Metal Cloak Overland Fenders, Uniden Bearcat 980, Homemade side rails. 50" and 20" light bars and brackets.