You're probably OK with the NP231. As long as the chain is in good shape (no chain slapping in 4WD), you should have no problem. Do change the oil though, as God only knows if the previous owner(s) did or didn't do so. So, when are you going to install a Tera 4:1 conversion...?
[b]'94 YJ Sahara Hardtop[/b]
4.0L / AX-15 / Stock NP231 / Stock D30 (open) / D60 (35-spline, Detroit locker, Moser axles) / rear disc brakes / 4.56 gears / Advanced Adapters SYE & CV Shaft / 33" BFG MT-KM's / Thorley Header / Magnaflow Muffler / BesTop Flares