If it's running good, why would you want to spend money on a tune up or having it looked at? Get a code reader and have it ready for when your truck gives you a MIL. Your Expedition will tell you soon enough when it needs attention.
On my '01 I did spark plugs at the regular change interval (COP's while I was at it) and run Lucas Injector Cleaner in a tank of gas every 4,000 miles. Only "tune up" work I ever needed.
There are a bunch of maintenance jobs that could probably use attention if you really want to get busy:
Doing the complete transmission fluid change is a nice thing to do for your truck. Pump out all the fluid using the tranny to do the work, change its filter and consider installing a drain plug in the pan during the job. Total DIY job and highly worth the effort.
Check the coolant condition. Anti freeze change is an often neglected service. Do as complete a change as you know how.
Check the brakes.
Consider changing the differential fluid. (Front too, if applicable)
Just a couple of suggestions off the top of my head and plenty to get started. The point is there's always neglected stuff to work on (read spend $ on) without attempting a tune up when she's already running good for ya.
