I just did my second drain/fill on my 02 rx, Amsoil syn. The dip stick says the diff requires a special oil. I called the dealership and got hold of a master tech, he said the mine did not require a special oil, but it changed in 03 or 04. I would check before doing the change. I drove about a 1/2 mile and then jacked it up again and drianed the back plug in question, it was full, about a quart of fluid.
The drain/fill is not that hard at all, I do the 2 plugs up front. Buy some rubber gloves it is messy.
check out this thread
http://www.clublexus.com/forums/showthread.php?t=220061