ok, offset is the distance from the center of the wheel (i.e.: if the wheel is 8" wide, the center of the wheel is 4" in from either side) to the mounting surface (i.e.: the surface where the wheel contacts the truck hub.)
the smaller the offset, the more the wheel will stick out from where the stock wheel is (there can also be negative offest which means theyre gonna stick out more.) i dont know what the stock offset is on our wheels, so i cant really tell you how far theyll stick out, but keep in mind that when you go with wider wheels, such as 8" wheels, the offest is gonna change, but that doesnt mean that theyll stick out, its just to make up for the extra width.
im also in the process of getting some 17x8 wheels, and i jsut called my wheel guy to let him know which ones i want, and hes gonna call me back with the specs of the wheel. i hope this helps a little bit - you might also wanna try giving EE a call - i dont think they sell anything that wont fit an explorer.