Let's see,..
For zombie(or any other kind of )apocalypse:
-Seriously low COG build. 2" lift maximum.
-Run-flat tires, maybe a custom rim that can take the Hummer H-1 wheel inserts.
-Roof rack delete. There should be nothing to get a grip on the top of the truck.
-Push bars on the front. Strong enough to nose up to a disabled vehicle(like a 3-ton pickup burned to the bare rims),drop in 4-low, and brute-force push it out of the way without damaging the nose.
-Some kind of armor on the windows. Expanded metal would do,.. something like the border patrol uses.
-Modified or shaved door handles. Maybe solenoid operated, whatever design used, anyone outside the vehicle doesn't have a hand hold or way to enter the vehicle.
-Oversize fuel tank. 30+ gallons.
(now for the fun part)
-Multi-fuel engine. Should be able to run any kind of diesel, kero, jet, veg oil, rendered pig fat, etc.
-Shielded electronics.(good luck with this one)
-Multi-band com radio.
-On-board potable water storage with pump and filter system.
-HVAC mods: cabin should be over-pressured, exterior air intakes filtered-at least HEPA, better if possible.
Probably should have a matching trailer:
-Trailer should be very low weight-less then 500lb empty if possible.
-Duplicate fuel and water storage in/under trailer.
Tow and trailer share tires. Trailer carries 2 spares, XJ carries one.
All gear/supplies storage split between tow vehicle and trailer. If you have to dump the trailer, you don't loose all fuel/water/etc. All the tools should be stored in the XJ.
Probably wouldn't be that great off-road. A solid trail rider, but too low/too much crap, too low on the nose to work well in rocks and hard trails.