OK, are you asking about vehicle prep or items to carry in your rig?
Most items have been covered - spare tire or plugs, tow points front & rear, tow strap (no metal hooks), jack, fire extinguisher, first aid kit, seatbelts, fluids, basic tools, etc... Then the next step would be things like spare parts, more advancced tools, CB or HAM, CO2 or air source, etc...
For rig prep, the XJ is very capable in stock form, but not what 99.9% of us have. To build your rig, you really need to decide what type of wheelin you want to do, talk to and look at lots of other XJ's, and finally budget. Most folks start with small lifts or buy a rig that has been lifted. Then we normally change the lift to a bigger one in a year or two... LOL. With a lift kit - regardless of size, you will need to ensure you have shocks, brakelines, driveshaft angles, tire sizes, speedo gear, etc... With larger tires you will want change your gears. And don;t forget the locker decision!
Then there is the whole armor thing. Gas tank skid and rock rails should be on the top of the list. Then aftermarket bumpers, diff guards, etc...
Ok, that's a ton of info and I hope I at least got some ideas flowing. It truly depends on you, your driving/wheeling style, type of wheeling, budget, ability, skils, etc..
Good luck!