I just bought a 99 expedition with the 4.6L with 88k miles. It seems to run great except when cruising along with the rpms about 1.2 - 2 if I give it a little gas it runs really rough... if I give it enough to down shift it takes off fine.. its just in that range. Getting new air filter today and I may pull out a plug to see how it looks. from the sounds of things around here the plug job will not be very fun.
Any ideas?

 

check for water in the plug boot. and if not water there then it could also be a plug going out

 

delong, you may need plugs replaced but you also need to have the coils stress tested because one or more are more likely the cause of the light throttle skipping.
I have just been through this specific kind of problem.
Replacing coils at random does not gaurentee a set of 8 good ones no matter where you get them.
If one new one fails, as put on, you go into a tail chase until it is found.
Pay dealer to do the test, then if you don't want them to replace any found to fail, it's up to you, at least you know what you have.
Come back and tell us what you have done to solve the drivability problem.

 

I have the same issue when I am driving at 55mph. Feels just like as you described. I think I also have a bent rim, but it feels engine related for the first part of the problem.