Well.... what are you having trouble with now that you need to go bigger/lower for?
Since you've already added the big axles, I don't see a lot of downside to a V8. Not much difference in weight compared to the 4.0. But it won't help any in slow speed 1st gear crawling over the 4.0.
Not a Jeep, but I've stayed 4 cyl and 100:1 gears. Not enough for my tasted, moving to 205:1 soon. Keeping the 4, but putting a little turd polish on it. As of yet, I haven't not been able to follow bigger motors through the mud, although hill climbs its underpowered.