Just keep it maintained, and you can drive an XJ as far as you want for as long as you want. Being older doesn't automatically make a vehicle worse, and being newer doesn't automatically make a car better. http://www.allpar.com/old/high-miles/richey.php
I'd trust my life in the hands of any number of older vehicles (Cherokee included) over some of these new models any day.
If you don't know its service history, change all its fluids (oil, coolant flush, transmission--don't flush, transfer case, differentials, maybe even power steering). Cherokees are SUPER reliable, and as long as you keep everything maintained (very cheap to maintain as well), they'll run forever.
1983 Chevy El Camino
1994 Cadillac Eldorado ETC
1996 Jeep Cherokee Country