Agree, its most likely the mud. In the front it can get caked into the rotors and any voids in the back of the wheels pretty good. If its still there after a good cleaning, check that you didnt knock off any wheel weights. If you aired down and did much bouncing to get over stuff, couldve spun a tire on the rim, too.
2013 JK - Metalcloak 3.5" coils and control arms, Bilstein 5160s, Prorock44, Synergy high steer, drag link, track bar and tie rod, PSC hydro assist, RCV front shafts, Reid knuckles, TEN factory rear shafts, 4.88s, 37" MTR, Tom Woods front and rear, antirock front and rear, etc.
spieg's build thread