Agreed. This isn’t something people do after the purchase. This is a feature that is designed into the truck when it is built. If your husband really needs a rear dual axle, he should buy a truck that has it.
If you look at commercial trucks, many professionals are converting from dual wheels to “super singles.” Super singles are single wide wheels and tires that replace the dual wheels. Perhaps your husband should find a shop that works on semis and look into having super singles installed. Check out some of these links:
http://www.diesel-accessories.com/page/index/super_single_wheels
http://www.bigtrucktv.com/item.aspx?environment_supersingles
http://www.ride-on.com/newsDetails.asp?nid=48
If your husband is trying to “keep up with the Joneses,” getting super singles instead of dual wheels will be a way to “one-up” them.