If it's only ran on the highway and not in the city they could last a LONG time, it matters a LOT on how it's driven, could be 15k miles or 150k miles... and unfortunately with the hydraulic clutches you get no warning, they feel the same with the hydraulic system working fine from day one to the day before they start slipping... If you treat it nicely you can limp it along for a while...
If you're doing the work and it isn't a huge inconvenience to do it twice, you could gamble, I've been there... The 4.0L clutch is pretty big so lasts longer... The hardest part is getting the transmission around the exhaust, they do the crossover in the wrong spot to be able to get the trans between the pipe and the trans tunnel without cussing a lot at least on the older ones...