After cleaning the harness and terminals on the sensor last night, the speedo worked all day today.
Many thanks to everyone who pointed me in the right direction! Trevor - thank you for making this thread turn from a "woe is me" to a troubleshooting endeavor.
A special thank you goes out to Hocrest for pointing me to the root of the problem. You rule! :)

Very interesting!
The service manual also notes that there are 2 speed sensors. When one malfunctions the other is used to keep the car operating. This would explain how I was able to drive without problem.
It also states one reads the front drive, and the other at the rear. If the 2 work in harmony to read speeds front and rear would they have some input into the transfer of torque in the AWD system? I noted no spinning of the front wheels while the speedo was out. Perhaps it goes to a preset torque split when one sensor is out? I'll have to do more reading to understand it further.

It would be a great help to the community if we could ship a set of the US market service manuals to you.

Good thinking Matt.

The two sensors are used as signal inputs to the TCU as you have suspected, and you are probably correct in assuming some sort of pre-set torque split should one fail.

It is also interesting that signals from the ABS sensors are also used. However I can only confirm this in respect of centre differential models, as is my JDM car. The centre clutch models could well be different in this regard.

As it happens svxcess has very kindly already sent me the US 1992 No. 6 electrical section and it has proven very useful. Special thanks are due because postage by air was not cheap.

As always, Trevor.