The first thing I would check is if there is a stick or rock that lodged up in between the engine/trans & skid plate.
My Trailhawk does the same thing. It wasn't too long after I had the car as well. I've been to lazy to look at it in depth. I do a good amount a offroading though, but it started happening after some easy offroading. Nothing hitting under the car.
I assume its the exhaust/cat banging around due to the rubber hangers being broken in. The heat/vibration has soften/loosened up the rubber hangers allowing for the extra movement.
Or it could be a hanger broke or a rubber hanger came off. I don't know for sure... too lazy lately.