This tutorial is based on a 1991 LS400, but same principle works through most Lexus models How To: Change Differential Oil
(please note images were lost in this guide so is now a text guide)
Keep the car on a flat level surface...slide under the car from the back and you should see this (bottom left silver bolt is the drain bolt and the top right one is the fill bolt):
Mine were on real tight so I had to use a breaker bar along with the 10mm hex socket to help break it loose...if you don't have a breaker bar you could just do this with an ordinary socket wrench...use your foot and "push" the wrench to help break the bolt loose.
Loosen and remove the fill bolt first, then get a container to catch the old fluid and proceed with removing the drain bolt. Once both bolts are out, wipe them off. The drain bolt has a magnet attached to it and as you can see in the pictures, all that black stuff on the paper towel were actually metal shards:
Replace the 2 washers, then tighten the drain bolt. Add the gear lube through the fill hole until it starts to drip out of the fill hole...I attached a little section of 1/2" plastic tubing to the bottle to help me get the new fluid in. Once the lube starts to drip out of the hole (approx 1.4 quarts) then screw the fill bolt back in there. Clean up the area and check for leaks: