With the help of the advice posted on this and this forum - I was able to quickly figure out that the culprit of simultaneous failure of the speedometer and trip computer was likely a failing transducer.

Replacement was simple remove and screw-in replacement. Part number for my Series III 86 VDP was DAC 4570 and WAS NOT DAC 4569 (which looks identical, is also a Jag speedometer transducer, but has a slightly different sized retaining nut). Swap fixed it all.

My electrical connector was up between those sheets of foam you see in the photo and I would recommend pulling the connector first before removing, replace with the new unit and then connect the new unit to ease installation and prevent twisting the wires unnecessarily

I took some photos to ease identification of the part under the car since that has been an issue for previous posters.






I also got some extra help on the car from my driveway mechanic here...