Zantus, sorry I can't help directly.
Alex, with any modification there is always a risk. I have heard several tales of woe, particularly WRT jeeps (which are the vehicles probably most commonly modified in these ways) where changes in diff ratio have gone wrong. Firstly, the two gears per diff that are changed during a ratio change are just about the most heavily loaded components in the drivetrain. Thus the quality, hardness and accurate fitment of these are critical. There is also apparently a required running-in procedure.
Further, any change such as this (whether a re-gear in the diffs, or a lower ratio in the transfer case) can apply more torque to all the components between where the change was done and where the tire touches the ground, and thus the risk of breakages is increased. This is dependant on traction of course. Having said that though, the Patrol is quite probably the car on the market best equiped to cope with these additional forces, and particularly the later cars with the 260 rear diff.
Personally, my preferance would be for a lower low ratio. I doubt a proper crawler gear is necessary for most applications, but even for my 33"-equiped car a lower low ratio (say 2.6:1) would at times be usefull. I would say a manual car with 35" tires would probably almost need such a change, particularly if rocks are a common obstacle (wouldn't bother if sand driving was my primary application), and particularly in the case of anything with a turbo (or your Lexus).
I would only bother with a diff-ratio change if going larger than 35", but others may not agree.
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