4x4, non off road package should already come with a stamped steel skid plate over the oil pan and "front" part of the truck.
4x4 w/ off road package gets the nicer aluminum skid plate and the transfer case skid plate.
I bought the transfercase skid plate for about $14 and added it to my truck.
TitanTruckz Technical Section and Knowledge Base - Installing a transfercase skid plate