Its skid plates, a 3.73 Limited Slip or today's trucks are "Locking Rear Differential". Generally it is a center shifter ****, but mine is a FX4 with the shifter on the steer wheeling... it has FX4 gauges. Also the shocks on FX4 are generally different then those on a regular 4x4... Now you can get them pretty much loaded anyway you want... I've seen F250's from the factory come pretty much base but have FX4 stickers on them meaning skid plates and all that.