Thanks for your judgement, Joe,
I've been in the garage for most of the evening and have new information. Right part and right effect but wrong cause.
When I got it apart, the WRD was free to move, however, the WRD housing was filled with rust and scale, causing an almost complete blockage of coolant flow around the thermostatic element.
The WRD responds to hot water from the tap, but it wasn't seeing very much hot water, if any, from the engine. That saves me about $900.00. The 230 SL part is about 15 times more expensive than the 250 and 280 parts. With hot water from the tap flowing by gravity into one of the tube openings, there was only slight weepage flowing out of the other. The guide rod in the control valve housing is very free-moving, so at least that's good news too.
The housing for the WRD is almost corroded away from the inside. My metal coolant tubes are also pretty rusty and scaly inside, so I've got a lot of tube cleaning and subsequent leak checking to do. I know that cleaning the rust and scale can create leaks. I've also found the wonderful but really complicated backflush procedure in the Technical Manual. I'll take my time and do that too, along with cleaning the fuel tank, fuel pump, and lines, very carefully. If the cooling system is that bad, I imagine the fuel system is too.
Maybe I got lucky this time.
Tom