You might want to look around the engine and drivetrain forum here (actually, the mods will probably toss this thread there shortly). There have been numerous posts on lockers and posi's over there.
I believe you can just get limited-slip from the factory with the TRD option. This would certainly help but isn't a true locker.
ARB does make lockers for the Tundra though, front and rear. They are about $650 or so per diff, and $125 for the compressor. Plus any labor charges. I've heard people state that it cost about $1800 to have lockers installed front and back
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