A little bit of info for you.....I see you are measuring backing plate to backing plate. That is not the correct way to measure the width of a differential (rear end). You need to measure WMS to WMS (wheel mounting surface). The backing plate method is not accurate at all because different differentials have wider brake assemblies than others. I would re-measure the differentials you currently have and see where you really are at. You are going to have to know your WMS to WMS measurement if you order a complete differential or just a housing.
A couple of places to look for a bolt in differential would be Currie and DTS
www.currieenterprises.com
www.drivetrainspecialists.com