Why more postively backspaced rims required with body lifts?

More postitive backspacing moves the rim's centerline outwards, making for a larger track - which increases the truck's stability. The stock rims are 5.25" whereas with body lifts, 3.75" is recommended.

It is not absolutely required, but a very good idea - especially on 3" body lifts. Lifting raises the truck's center of gravity, and increasing the truck's track compensates for it.