So I got a salvage throttle body and sure enough that did the trick. I now can control the idle. I still think it is mysterious that nothing I tried could identify a vacuum leak. Oh well.

Darrell....In your case there likely was no vacuum leak. The idle air control passage inside the bottom of your old TB had worn out and was likely letting max air possible through that bypass. I had the same thing with my 89 3.0 and just recently got a wrecking yard TB that solved my problem also, but I have no confidence that it will last.

Has anyone opened a TB of this configuration and thought about designing a work around that would provide adjustable air bypass. This bottom passage could supply the minimum idle air required for a very slow idle, so that one could then adjust the idle to spec using the top side of the TB adjustment screw to speed the idle up ( by backing the screw out)?

The engine may be cold blooded by going that route but at least it would be happy when it warmed up. The drivers and the brakes would be happy also!....:-)