Is there something wrong with the axle that's under there right now? Either one of those two axles aren't really going to gain you much in the strength department over that stock axle.
The V6 rear will have essentially the same diameter axle shafts and will have a centered rear end, but a smaller (~8.25) ring gear. Not sure if you would gain any width over that stock one (if any).
The J62 rear axle will have a RH diff offset with an equal size ring gear (~9.5). The shafts will be eqaul in diameter. You wil gain about 2-3 inches total width over that stock axle.
Of course, that is if that is a BJ, HJ (did they even make H or FJ series. It looks like a 74, and I think I can see leaf springs, so I'd wager it's a BJ74, so that info applies. But, if it's an LJ or PJ (light duty LC with coils), then it will have an axle pretty much equal to that of the V6 rear and a J62 would be more (but not really that much) of an upgrade.
Maybe I'm missing something?
-Ferg-