Due to my additional weight of camper shell with roof rack, hitch tire carrier with 35" spare, and other 'goodies' inside the camper shell, I definitely noticed a slight sag on my 02 2500hd. The shop added a 'add a left' and it has worked out great. No more sag and I do not feel any difference in ride quality. The shell, hitch carrier, etc. are always on maybe that's why I don't feel any difference and truck is never 'unloaded' to know otherwise. It's a small investment and less moving parts compared to say getting bags. Parts and install for me was $130.
Not sure if I need a whole new leaf spring setup. I'm sure the shop would have told me $$$$.