I had a problem with my '98 tour with the CEL coming on on the Highway as well. I had Ford look at it and they said was an o2 sensor, they said that the next 3 times after that then the mechanic ordered a new computer and the new comp started to report a small $15 part (can't remember the name) but what tipped the mechanic off was the the forward bank of cylinders had nice normal brown covered spark plugs but the rear bank was black excrusted (running rich) so if you don't have a code reader or if it tells you that it is an o2 sensor then check the plugs as well.
This may have nothing to do with your problem but I always like to share different possibilities.
