The RXh actually has a ~75HP electric (extraordinary low speed torque) drive motor dedicated to rear drive whereas the RX350 has a somewhat fluid viscous clutch/coupling for rear drive that mostly remains "flaccid" until a quite significant level, certainly tens of milliseconds, possibly hundreds, of front wheelspin ocurrs.
The whole matter gets a bit confusing if you start to look at the history of the RX's AWD system.
Up until 2001 a viscous clutch was used, mounted across the two outputs of the center diff'l, along with a mechainical LSD at the rear. A totally non-functional LSD since seemingly there was/is no way to get enough drive torque to the rear to result in rear wheelspin/slip. The fronts, recieving the clear majority of engine torque, at least at the initial instance of onset, would always be the first to develop wheelspin/slip and thereby significantly limit the engine torque level to the rear, but overall in reality.
In 2001 VSC/Trac was added which mostly made the viscous clutch redundent (it was likely non-functional to begin with anyway) since the instant front wheelspin/slip was detected Trac would apply braking and dethrottle the engine. The mechanical LSD was also dropped in favor of a virtual LSD via use of Trac braking (Yeah, like that's likely to happen). On a 4 wheel dyno the most we could get from my 2001 was a 75/25 F/R torque split and that was only by disabling VSC/Trac (pulled the ABS fuse) and ~10-15 seconds of front "wheelspin" (overspeed vs the rear) developed by dyno braking of only the rear wheels. Otherwise it seemed to meaure ~95/5 F/R, if even that.
Part of the mystery for me, but at the same time something of a confirmation of my theory, is that the use of the viscous clutch was discontinued for the entire RX330 product run but is apparently back in play/use for the RX350. I can only assume the Trac firmware was modified for the RX350 to provide a Trac dethrottling delay as they now have on the Trac system for the RWD Lexus models.
While I'm stuck here at Eastside Group Health ICU for a least another day I'll likely be back at the office by Wednesday. Our offices are located directly behind the Redmond Brown Bag Cafe if you would like to drop by sometime and I can explain in far more detail, c-best options, flawed climate control design, hazards of FWD, 1-2 second engine/transaxle delay/hesitation, etc.
I appreciate all the information, WWEST - I'm a lexus newby so need to absorb it all...may be I'll see you around...