Well, Fire Fighter, although you are in the YJ tech section instead of the TJ tech section, my answer would be NO. The reason is, a New Process 231J t-case has a pump and filter inside of it, and the heavier oil will cause problems with it. Just my 2 cents. I replaced my fluid with Royal Purple synthetic ATF 4.
5W30 syn. is thin, but the viscosity isn't the same as ATF. Some run regular oil in their t-case, but does it hurt? Couldn't answer that truthfully.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...