Yup, and it seems like the newer Rangers are more prone to it too. I had the camper on my truck for 7 or 8 months right from day 1 when I bought mine new, 18 months later and it sits over an inch low if I dump my airbags (they stay at 30 psi to keep them at height). My roommate's '07 FX4 Ranger is sitting almost 3" low on the one side just from the canopy and camping gear sitting in it for the last nearly 4 years, it's getting new rear leafs already.. They do sag for sure, IMO install airbags if you're going to be hauling any serious weight to save your leaf springs. LR
'09 Ford Ranger FX4 that gets abused a lot... but doesn't like to swim though.