Having taken my XJ on long trips over dirt roads, rocks, highways, etc I have (so far) come to the following:
Mechanical breakdowns happen, and I don't need to try and carry spare parts for everything. Only time I got stranded? AC compressor blew up. Did it give any signs beforehand? no.
Skid plates. Run a beefy transfer case skid plate. Leave the stock front skid, if you have one, intact. Add diff protection if you want to or if you play in the rocks. Gss tank skid, too. Factory is fine.
"LT" tires. I've run LT 31s, now I'm back to P series 235s. I like the P series 235s - they didn't let me down while in 4-low on my way up to the camp site, 45 degrees nose-up in rocks and loose gravel. You don't need any bigger than a 31 for "expedition" travel, and probably no bigger than a 235/75/15.
Brakes: XJ brakes could be better. I run ZJ disc and like it. WJ will probably be better still, but it isn't needed.
Suspension: upgrade hardware size on the axle end track bar mount if it wallows. Otherwise, choose high quality components when you do your routine maintenance. The stock XJ suspension is just fine for 3 adults, a dog, & a whole lot of climbing gear over a rough trail even at 14 years old. Rubber > poly for ride quality & noise.
Fuel economy upgrades: the longer you drive your XJ, or plan to, the more important these are.
Finally : be proactive about your repairs. You can hear the starter solenoid hanging on an extra second or two? New starter. Only charging at 13.4v? New alt. Belt is cracking? new belt. Etc. Don't let things get "sketchy" and they won't break on you.
Roof racks: avoid if you can. Small trailers or hitch-mounted trays are better. Aerodynamic roof boxes from Thule or Yakima are better than big open baskets.
There we go: that was pretty much everythign I know about exped. travel in an XJ.
