Just a warning: Never, EVER, buy tires from Wallyworld... why, you may ask ?
Cause they are shit, bottom-of-the-barrel crap that were made exclusively for them, to reach a certain price point, and cutting whatever corners necessary to get there... been there, done that, never again !
My son bought some of their cheap Goodyear's for his car, which are normally really great tires, and they were supposed to be 65k mile tires, which ended up lasting about 22K miles....
Then when he went back there to get them replaced under warranty (prorated of course) they refused and said F you, not our problem, contact the mfgr....
Unlike my SR-A's on my 2011 EL Limited, which I have put over 50K hard highway & construction site miles on and they still look almost new ! But of course I got them from my favorite tire shop, who, at the time of purchase, made it clear that if I managed to wear them out before hitting the rated mileage, just bring 'em back for a pro-rated replacement !
Anyways...
YES, the diameter of you existing wheels will dictated what size tires you can use, unless you plan on replacing them too...
And on most Expeditions I have seen, including mine, there should enough room to move from an 18" to a 20" tire & wheel, assuming of course, that you stick with the recommended sidewall height and tread width too, and that your vehicle's suspension has not been lowered or altered downwards from the stock specifications...
Good luck !
And just an FYI, Goodyear usually has some pretty good rebates on their tires, ranging from $100-200 per set of 4, which you get back in about 30 days via a visa debit card, if you submit your claim online