I have the 99 Expedition 5.4 Edie Bauer edition with 130k. I recently took my truck in to have to oil changed before a long trip out to the Mountains. They told me that I had coolant in my oil and oil in my coolant and chances were I had to get my head gasket replaced.
I took my Expedition to a reputable mechanic and had them take a look. They said I needed a new head gasket and that it was making a loud knocking sound when it first starts up cold. (I live in colorado so the temp does go below 25 degrees from time to time) They suspected the main bearing was on its way out and wanted to replace my engine for a rebuilt ($3k).
Bummed as I was I felt I had to get a second opinion so I took it to the nearest Ford dealership.
After a couple of days they called me back and told me my engine was running just fine and that I didn't even need to replace my head gasket. The only thing that was causing the transfer between the coolant and the oil they said was some adapter. Not sure what it's called but on the parts sheet it says AA adapter.
In all it cost me just over $500 bucks.
No more problems for me.