The EEC.. or PCM as it's now called... is the same computer for the engine and tranny - the tranny doesn't have its own computer.1st thought is VSS ( vehicle speed sensor).
Hit up the local Ford dealer and talk to a tech there.
I am not positive but I don't think a VSS will not send out a code to the ECC because the trans has its own computer that gains all info from the VSS and the TPS.
Honestly, i've never moved the plunger by hand, but i think if you look in there, you'll see a spring, which makes me think that if you're just pushing it against the spring, it would normally move some. The IAC won't be the problem anyway. Sticking IAC valves most commonly cause idle concerns and stalling, not what you're getting, since the throttle will be open as you're driving down the road, which makes the IAC useless at that point.???? one question, took IAC off to clean and inspect. plunger moves 1/4 inch freely with a little resistance. good or bad?