Depends on the kind of driving you'll do. A normal (or 'open') diff gives
all the torque to the wheel with the least traction. So, say you have one
rear wheel in pavement and the other side in mud, the wheel in the mud
will spin and spin and the wheel on pavement won't budge. An LSD prevents
that from happening, transfering more torque to the side that has the most
traction. www.howstuffworks.com has great explanations of how all this goes
on.

In 2WD, a LS diff can create some fairly undesirable handling problems on
ice or mud. In 4WD, it isn't so bad, but can still be a problem in the wrong
hands. So it really depends on the sort of driving you do most of, and the
amount of finesse you are capable of contributing to the handling of the
truck.  A push-button locker has a lot more value.

if you are looking for some extra traction on occasional ice or gravel
then go for it ... It will wear out over time.

You will be better off getting the electric locker from toyota

Don't all toyota 4x4s come with rear limited slip diffs?

Absolutely yes, there is a huge benefit. Get the limited slip and don't look
back. Run, don't walk, to the sign up desk and get yours today.

Without limited slip (if you can get it in both axles, this is even better),
you get drive torque to both tires on the same axle ONLY WHEN THERE IS GOOD
TRACTION. When traction takes a bye under one tire, the Open Differential
(this is what they call it when there isn't a limited slip) will divert all
engine torque to the tire that hasn't got any grip, the limited slip will
send torque to the tire that still has a chance of moving the car/truck
forward.

Get the limited slip, and don't give it a second thought.

Another option is to get the TRD, which has a locker. The locker physically
locks the left and right halves of the axle together, and this means both
tires get full torque no matter what the traction is doing. One last option
is the TRD with the Limited Slip. This is the most costly, but well worth
it - especially if you intend to offroad.

I installed a LSD in my '01 Tundra so it would have better traction
on roads where I wasn't using 4wd, in the rain, for example.  Also,
I've been parked in deep snow and had just the two wheels on one
side spin.  With rear LSD, I'd have the rear two pulling almost equally.

Ken

If you have an open differential, the torque applied to either rear
wheel is limited to the the torque that breaks one of the rear wheels
loose. If you are on a consistent surface (level, equally slippery on
both sides, the truck is uniformly loaded, etc.) this is not a
disadvantage. The two rear wheel will break away with approximately the
same torque applied, so you aren't loosing much. However, if you are on
an inconsistent surface (one side slippery, or lower, or have an
unevenly loaded truck), the maximum torque to either wheel is still
limited to the torque that can be applied to the wheel that willl break
away first, which, in this case, can be a very low value. A limited
slip differential locks the two axles together so that the two axles
spin together instead of letting the wheel on the slippery (or lightly
loaded) side spin freely. In this situation the torque that can be
applied to either wheel is no longer limited to the torque than can be
applied to the wheel with the least traction. This is good when one
wheel has good traction. However, if both wheels are on a slippery
surface, it is very easy to break both wheels loose if you have a
limited slip. This is bad becasue once both wheels have broken loose,
they no longer will maintain the lateral position of the vehicle. This
means the rear axle will most likely kick out. If you are on any sort
of angled surface, the rear end will slide down the hill while the
wheels are spinning (and once started sliding might not stop, even if
you quit applying power). If you have 4wd drive, the front wheels will
tend to pull you along and keep your truck pointed in the right
direction (probably yawed somwhat). If you only have two wheel drive
with a limited slip, you are likely to find you rear end slidding down
the hill and end up with the front end pointing up the hill and be
worse off than when you started.

The worst thing you can have on a really slippery surface like glare
ice is 2WD, limited slip, and traction control that works by limiting
engine power (and doesn't use selective application of the brakes).

A manually lockable differential is the best idea. You can select when
you lock the rear axle and can therefore avoid loosing lateral location
of the rear axle. However, on a consistent slippery surface, this is
not going to save you. I have been driving farm tractors for most of my
life. All of the tractors we have purchased in the last 40 years have
included a manually operated rear differential lock. I have never once
had this feature be of any use in a muddy field.

Traction control that selectively operates brakes on opposite sides of
the vehicle while modulating the throttle is also probably a useful
feature. I've never driven a vehicle with this feature. Some high end
tractors actually include ground speed radar and modulate power to
limit slip to the most desirable value.

I see that Toyota is playing up traction control for the Tundra. Anyone
have details of how this sytem works? The TV commercials seems stupid,
hopefully the actual system is a good one.

Ed

Have a Detroit in the front/rear of my toyota pickup, open front/rear in
my FJ6x.

It depends on how you drive and where you go.
Limited slips are for pavement and dirt roads.

Serious 4x4 trails and rock crawling is Detroit/ARB time.