Ok,        first you need to understand that "lockers"
and "limited slips" are 2 different animals.
A locker will provide power to both wheels and still allow you to negotiate
a turn.
A limited slip unit provides power to the wheel with the most traction
(hopefully) and is fine for general trail use but when the going gets rough
you want a locker.
Tractech makes automatic lockers and ARB makes
the air locker for your truck, I don't recommend the ARB
because of horror stories i have read about them.
Anything from compressor failure, air-line breakage,
switch malfunction plus the added weight of the peripherals.
You can't break a detroit unit, there totally automatic
set it and forget it units plus it costs less than the ARB.
i would'nt even consider a electronic unit if i anticipated
frequent water crossings.
Check out tractech:
http://www.tractech.com

check out my truck by clicking the link below my name

TracTech does make an electirc locking limited spli (the ElecTrac I
think its called).  It is only avlaible for a few applications, such as
the common Dana axles currently.  No version available yet for the
Toyota 8" axles.  ARB or the Toyota e-locker are the two manually
selectable options:
    http://www.4crawler.com/4x4/4R_TechInfo.shtml#DiffInfo