Factory Toyota G series differentials are pretty freakin stout. My buddy does LS swaps for a living and currently has a LS1 tuned on E85 making 380 wheel horsepower in his 89 Cressida with a G series diff that he drifts almost every friggin weekend and doubles as his daily driver. People don't understand that most of the time, differential damage is caused by one leggin it. Your spinning that one wheel so fast and there goes your spider gears. You have to go with a T1 or T2 torsen limited slip differential or a locker or heck, even a detroit locker when it comes to doing burnouts and stuff in order to keep your diff from going bang.
A LS is smaller than a Toyota V8, so fitting it in the engine bay of a 4Runner is seriously no hassle. In a Toyota V8, you got 4 overhead cams vs a LS with a single cam in between both heads. That alone shrinks the engine big time. Since your even considering this swap, why not go for a T56 instead of the weak sauce 4L60E?
Here's pictures of my buddies swaps on his personal vehicles.
LS1 in his Toyota Cressida:
1UZ-FE V8 5spd M/T in his Toyota work/drift truck:
I mean, if you really want to keep your 4WD if you have it and want to go V8, you can always pull a low mileage Lexus LS400/SC400 V8 out for a few hundred bux with the tranny and swap your transfercase on the Lex Tranny or put your 4Runners tranny on the 1UZ bellhousing and not worry about making custom driveshafts which you will have to do with a LS motor. The first gen 1UZ V8 was an engineering monster. This thing has 6 bolt main bearings, fat rods and low compression for boost. You could literally spend the same money on a boosted 1UZ swap as a stock LS swap and get way more power out of the 1UZ reliably. Just sayin..