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Don't know specifically on an Expedition, but in general for someone who is mechanically inclined, installing a cooler is fairly simple, I would guess a 1 hour job if you take it easy.
The key thing is to find a good spot to mount it. You want good airflow, not a lot of vibration, protection from rocks, don't cut off flow from something important. One word of warning, you will probably need bandaids. Everytime I do this(3 or 4?) I always get small cuts from the radiator fins.
Depending on how your truck is setup, you might find the job easier to do if the grill is removed so you have access to the front of the radiator. I like to keep the cooler a bit high so it doesn't catch any rocks. Make sure the hoses don't crimp or rub against any sharp surfaces. Make sure all your connections are snug, ATF can find any little leak.
It's really very very simple, the worst problem I had was scratching my knuckles on the radiator fins, and getting the hoses not to crimp. I would go with a stacked plate cooler design its alot better than the other ones for its size, also the mounting pins that come with the Hayden coolers? They stink, you might look into getting some steel wire or something to use to mount it cuz those plastic pins just didn't cut it.. it took me less than 30 minutes, you should be fine!