I spoke too soon....
This morning it happened again. Car has been dry since I did the vapor barrier fix.
Conditions: 7:10 am CST, 40 F outside, overcast, dry road. Driving around a left bend, banked, at 65 mph (yes its one of the more exciting parts of my commute to work) with the DSC turned off. I goosed the accelerator a bit which coincided with the car going over a slight rise when I felt the inside rear wheel slip slightly. Then the engine rpm's went to 5000 rpm and the trans fail safe message appeared on the dash.
I coasted to a nearby parking lot, parked the car, used the stowable jack and jacked up the driver side rear tire. Put the car in R and then D and watched the wheel turn.
Put everything back and drove on to work.
I am pretty sure it is not the transmission per se. Maybe the control unit or a sensor. I am thinking its rear diff related. But that is still guess work.
2003 BMW 540i A M-Sport with factory Navigation and CD changer; winter package
1997 Toyota Landcruiser 40th anniversary edition