personally, i am not a fan of diff guards, they can pick up stuff & cause more damage than they are worth.......i have never bust a cover.......yet!!
transfer case skids are relatively easy to come by...........i could well be breaking another early XJ soon, so if you want a skid, let me know & i will put you on the list!!
fuel tank skids are another matter.........i know surrey offraod used to do em, cos i had one.........again, the early XJ's seemed to have a much tougher fuel tank cover than the later ones (after 95 i think they were made by cadburys!)