DLeno, backspacing is distance from the inner rim edge to the back of the mount pad. Most wheels are around 4. 5" to about 5" for wheels in our size range.
I have some 18" Weld wheels with +10mm offset & 5. 25" backspacing and they stuck out wayy too far when I tried one on my '01 2-wheel drive. With bigger tires and fat fender flares you should be OK your 4x4.
Vaughn