One other thing, my wife's '97 Cougar 4.6L I recently did a tuneup on (and later an entire intake manifold because it cracked).

I checked the throttle body, and found enough corrosion in the throttle body bore where the throttle plate is that the plate couldn't completely close while the actual air-flow was cut off from the corrosion.

I know it's supposed to be "coated" with something, I cleaned it out with Scotch-Brite, not giving a darn if the coating was coming off, because the corrosion had already taken care of that. Runs better now. But, it didn't have a high idle like you're complaining about, but it has only 22K miles on it. Yours could be much worse than mine.

Also, check for obvious throttle cable binding, where the throttle plate isn't closing all the way.