As was previously stated, the ONLY 4Runner that had a rear locker was the 09 Trail Edition. ONLY came in a V6. That doesn't matter though since the axles are the same. That being said I'm sure you could swap an FJ or Taco rear end as long as the ratio was the same. The thing that would really suck is that you'd need the complete rear axle, wiring harness, and possibly an ECU in order for it to work. I normally like the idea of keeping it all Toyota but I don't think it's worth the time or money. I was curious about those rear axles (we have an 08 4R) and so I did some looking. The only one I could find online for sale was $1750. And that was just the axle.
So, this is def one of those times where it's easier, and probably cheaper in the long run, to just put in an aftermarket locker like an ARB or A/M e-locker.
So, this is def one of those times where it's easier, and probably cheaper in the long run, to just put in an aftermarket locker like an ARB or A/M e-locker.