Any work you can do yourself will save you a lot of money, provide you know what you are doing.
Have you hooked up a scanner to see if there are any codes? In the US, most auto parts stores will run a scan for free and print a list of condition and solutions if applicable. A scanner is as valuable tool, if not more valuable than any tool you can get.
I am pretty good with cars but also, I am aware of my limitations. Throwing parts until one solves the issue is not a good way to approach the issue. Your best bet is to take it to a competent mechanic, familiar with the Expedition, and with the proper equipment, get an accurate diagnostic and go from there; in the long run it will end up being more efficient and much cheaper.
"If you think hiring a professional is expensive, try hiring an amateur." - Red Adair