The tranny Skid covers the oil filter.
It would seem like the engine skid would do that, but the Mopar parts catalog and names are not always helpfull. The salespeople make it even more confusing. I looked at aftermarket skids to get pictures of each skid and where it went. One site is here:
http://www.skidplates.com/products_02_KJ.htm.
You can also look at bolderbars.com The pics on this site show several pictures of each plate. Cross referencing these with the equivalent Mopar plates, you get the following:
From the front to the rear:
Engine/Suspension skid: covers bottom of suspension and the front of the engine. Most of the engine (CRD) not covered, including oil filter. Came on my CRD, but flimsy compared to my aftermarket tranny skid. Catches lots of dirt under it if you bottom out the front end in the sand. When I loosened it to put the tranny skid on, I could have planted a tree in all the dirt that came out from under it.
Tranny Skid: Covers rear of engine and transmission. Starts where the front engine skid plate ends and goes back to the transmission mount cross bar. Covers the oil filter. (Bought aftermarket one. More $$$ than Mopar but thicker. I don't know why this is not stock with that filter down there....)
Transfer Case Skid: Covers transfer case. Starts where the tranny skids ends at the cross bar and goes back at an angle to mount to one side of the frame behind the transfer case.
Gas Tank Skid: Covers the gas tank only. Stock gas tank is plastic/plastic covered. My friend with an XJ does not have a skid here and he occasionally bangs his plastic tank. The metal inside the tank dents inward, but the plastic outer shell pops back out-- so you can't tell if you dented yours or not. I am not sure if the KJ's tank is the same, though. With his XJ he noticed that he could not put as much gas in it after he banged it up. Mechanic took it off the Jeep and pounded the dents back out of it and now all is well. Good enough reason for me to look into one of these, as taking the gas tank off is not cheap/fun. Some say Mopar gas tank skid will not go with the factory hitch. Some aftermarket ones will, but they are more cash. The hitch offers some protection anyway, but not much-- as friends XJ had a factory hitch and his tank was dented anyway.
Next for me is tow hooks and transfer case skid... The tow hooks will be Mopar, the transfer case skid has yet to be decided. Then boulder bars and gas tank skid.