1999 Ford F250 4X4 Diesel - fuel leak only at idle?

I assume that the dripping is coming from the engine area? You need to look under the hood to find where the leak is coming from. It could be connections of the fuel line, it could be the fuel filter cover, or any number of other things.

It looks as though its coming from a 'yellow" valve near the fuel filter, and also it looks like the fuel line just slips in...there is no clamp on it. Do you know how it is tightened or should there be a clamp?

I will look at it tonight when I go home.

There is nothing on my 2000 7.3 diesel like what you are talking about. If indeed your leak is coming from that item, can you see whether it looks like a clamp would fix it or is it threaded? Usually the fitting of the fuel lines are metal lines that are compression fittings. So is there a nut that you can see on that line?

I talked to my dad in Utah, he said that possibly the "fuel separator valve" is leaking. Unsure if that is the valve I'm talking about, does that make sense? Do you know, what the fuel separator valve is? He said it separates the water from the fuel and sometime under extreme weather conditions will fail and leak.

I think what I would do if I was you is this. Ask yourself if you would do the repairs? If the answer is yes, take it to the auto parts store and have them look at the part in question and they can advise on getting the part. If the answer is no, you can take it to the repair shop and point to the leak. Either way I trust you are going to get the item repaired and have a great trip to visit for Christmas. Merry Christmas!