The rear differential gear ratio on the 1998 -2000 LS400 and 2001 - 2003 LS430 is 3.27 : 1.

Both use Aisin A650-E 5-speed transmission with .753 fifth gear.(but different torque converter P.N.s that no one can answer as to whether differences)

The rear differential gear ratio on the 2003 - 2006 is 3.769 : 1 and uses A761-E 6-speed transmission with .586 6th gear

See:

https://en.m.wikipedia.org/wiki/Toyota_A_transmission

Now the question;

While the differential appears to be a bolt in proposition, does anyone know whether swapping the necessary transmission and speed sensors is plug-and-play practical?

Correct speedometer, Engine, braking and traction control functions important.

Thanks

 

No because the computer will need to be told what wheel rpm is correct and if you don't it may see transmission shaft speed and apparent wheel slip/under speed and throw VSC errors. It depends on the car and I've never messed with axle ratios on German/Japanese luxury cars, only on US muscle and off road applications.

You may want to try it and see what happens since it won't harm anything mechanically, you may be able to adjust for tire size and if you can this becomes insanely easy since you can just fool the computer into thinking you have really small tires instead of a totally different ratio. That way it will display correct speed, and al other functions since according to it the tires simply changed size, that method is used on dodge vehicles when we swap them to 4.56 and up gears