how much beefier is the drivetrain of the fj The unlimited has the 4.0 which is better than the fj engineMy dad would even admit to this. the nsg 370 seems to be a nice tranny may not be as stout but you have more in the way of gear selection. np 231 is a solid transfercase yes there are better. And lastly the unlimited come stock with a rear dana 44 I dont think the dana 30 axle is suppose to be that bad correct me if I am wrong. i do wish I had a fj40 they are really cool I am still mad at my dad for getting rid of his it had the rusties.
Consider this, FJ'ers don't really worry about axle swaps unless they're turning 38.5+. They do require some front axle upgrades but the Ring is around 8+" to begin with. They are larger then a standard TJ which is nice.
The stock engine is a 4.2L I believe. Not to be confused with the Jeep 4.2L. You can upgrade it to a new I6 turbo though. Now that would be cool. No smog in CA since most are older then '75's so even in CA you could do whatever you wnated.
Honestly, if I was going to trade I'd go with a Early Bronco though. Lots more room and a V8. D44 and 9" axles already so 35's are a easy swap.