I have an OEM gas skid that fits perfectly with the hidden hitch. They are both held in place with the inserted nut rails. I do all my rear jacking under the skid and to date have not seen any damage.
By-the-way, two months ago at a salvage yard in Waterbury CT, I saw an early XJ with both the front and rear OEM skids in good condition. I do not remember the phone number but the yard is known as 'Johnnys Auto Salvage'. Actually, they are North of Waterbury off US8.