Frontiers come with the spring over rear axle (unlike the Xterras spring under). You could theoreticly take the rear setup from a Frontier and install it on an X, BUT to avoid the 6"+ of lift, you will have to use the Frontier leaf springs because they are flatter.
On the frontier.. Was this done to aid in loading the pick up bed? So it doesnt sag as easily? Some stock frontiers look taller than lifted X's.
Check out the NOR board, this was already covered.
L8r