assuming your talking about a front one? i dont see there being any benifit of swapping the rear its not very big and cant weigh much for the back.
man bill really needs to change my name...........FML
you CAN replace the rear bumper support beam with the alum aztek/redezvous front bumper
you have to cut off and bolt on the steel brackets from the OEM rear bumper beam but you can drop weight that way