I had a similar problem with my Ranger although it was coupled with rough idle on cold starts. Also, the High Idle problem only occurred during cold temperatures...not during the summer.
For the low idle problem, I had leaky Intake Gasket Values (see Ranger Post
I just got the truck back and it appears the High Idle problem has diminished greatly as well.
You may need to have these gaskets replace OR
, it could be a vacuum leak some where else in your system (vacuum is anywhere
in the Intake System (lines, hoses, manifolds, etc.))
I would recommend having Ford diagnois the location of your leak (probably $90 bucks) and then if you know someone that is familiar with cars or if you are comfortable with them you can possibly fix the problem yourself without paying hundreds of dollars in labor.
Hope this helps