We had the exact same experience. Our Lexus just hit 100,000 miles. On the way to a mall, a strange sound came from the transmission. Parked the car, and game over. The car would not move, nor would it budge in neutral. The flatbed had to drag it - tires screeching, onto the truck. Our Lexus has always been well taken care of - and I expected it to last a long time before a major failure. I must say I'm very disappointed.
When the 2001 RX300 came out, Lexus had adopted TC, Traction Control, across the product line, FWD and F/awd models. VSC was also included (option..??) for 2001. With TC, the VC then has no real practical use, so the formulation of the viscous fluid was changed such that it almost NEVER provides any reasonable level of torque coupling to the rear wheels.
Some of this now appears to have been done to help alleviate the premature transaxle failure occurring in the '99 and '00 RX300's.
For the RX300, Lexus had developed a more robust form of the Camry transaxle, and in the process developed a technique wherein real time control of the ATF line pressure was available. The old technique, constant pumping of ATF with a high pressure spring loaded relief valve, and a line pressure holding/sustaining accumulator, was abandoned (Lexus uses the word "abolition").
The new technique not only allowed more room within the transaxle case for more robust components, it also improved FE not insignificantly.
But there was a fly in the ointment.
With no pressure holding accumulator, there was no way to sustain enough line pressure with the engine idling if a second shift was required in quick succession to the previous one.
Lift the throttle for a quick coastdown to a lower speed, the transaxle upshifts, or at least begins to upshift. Now quickly apply throttle for (re-)acceleration, the transaxle will NEED to downshift but with the engine now at idle there is no line pressure to accomplish that downshift.
The engine revs go up in the meantime, and you will soon have burned clutch frictional surfaces.
In 2001 they thought they had overcome this problem by changing the F/awd system and having the rear time ATF line pressure "hold", ALWAYS hold, the ATF line pressure at a higher level than was specified in the original design.
Localized overheating of the ATF resulted, and so the 2001 and later models of the RX300 suffer from burned ATF in as little as 40,000 miles.
The owners manual states that no ATF scheduled maintenance was required because that was the PLAN when this model was shipped.
Obviously those plans went awry.
For the RX330 model series, DBW was adopted/used to "protect the drive train". If a downshift cannot be quickly accomplished, DBW is used to delay the engine from rising in RPM until the downshift is completed. The now useless VC was also dropped entirely.
Thus we now have the infamous 1-2 second downshift delay/hesitation to deal with in the DBW models.