I was wondering what size tires i can put under my Expy its 4wheel drive with 5.4 liter and stock I want to put 33 by 10.50 by 15 if this does work what width rim and what should the back spacing be, i dont plan on much 4 wheeling all I want to do is fill up the fender wells

 

You won't be able to find any (if any at all) 15" rims that fit the Expedition. If you do find some with the correct bolt pattern, you'll have to do a little grinding on the brake calipers to get the wheels on.

As for tire size, you can go with a 285/75R16 and fill the wheel-wells nicely.