How hard is it to add a diff lock to a 4wd 4runner that didn't originally have the option?

Locking diff was offered as an option on 3rd gens. I assume it would be easier since it was offered when it was new.

 

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Easiest way is to swap the entire rear axle with one that has a rear locker.

Non-lockers are 8.4 whereas lockers are 8.0 so you can't just swap out the 3rd member.

 

I just went through this, kinda. I blew the rear axle seals on my rig and I had my shop throw a rear locker in while they had it all apart. For my rig, the sway bar did not clear the elocker motor and I believe there is some trimming/grinding on the axle itself in order to make it all fit together. If you are going to do it yourself then I would go with swapping the axle out for one that already has the Elocker. Totally worth it though... IMHO