Most 4 cylinder YJ came from the factory with 4:10 gears making this an easy swap for those in search of lower gearing. My buddy runs a D30 in the front of his YJ with 4:88, 6cyl and no locker and runs 36" Swampers on a daily basis and has not yet blown the front out(I know, I know he's been pretty lucky so far). There are also lots of other upgrades to further strengthen a 30 but it would be wiser in my opinion to build say a D44 if you wheel more than twice a month.
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80 Cj5, 360amc, Swampers, Custom 5in. lift, Off Road only
92 XJ 6" Rubicon, 32s, strictly street