Switching a 2004 2wd 3500 silverado DRW to a SRW

I wanted to switch my 2004 Silverado 3500 DRW 2 wheel drive over to a SRW and have started the project. The front was easy to do by removing the hub extender that was bolted on to the inner studs. The rear is not quite so easy. I installed the new rims and my rotor is not recessed in the rim. I measured from backing plate to backing plate and this measures the same as a single rear wheel rear-end but the hub extends out further and was wondering what I need to replace to make this work? If anybody can be of help post a response or email me at mesh75@wittenbergnet.net