Basically, what we put in, and not on, our XJ's.

Here is what I carry all the time in mine.

Flashlight
Duck Tape
Extra fuse's
Extra wire
Blankets
Emergency flare
Wide Base candle with matches
Jack
Four way tire iron
Basic car tool set( sockets, wrenches, screw drivers, wire tool)
Zip Ties
Ice Scraper
Recovery strap
Ratchet Straps, 6

Most of this all fits in a nice heavy duty rubber made box that stays in the back.

This is just what I carry, what do you consider "basic necessities" in your XJ.  Either add to this list, or make your own.