Rear End Sag

Rear end of the truck sags more than I like when pulling a trailer (small single axle trailer with a four wheeler on it) or when I have the four wheeler loaded in the bed of the truck.

Is it possible / recommended to replace the rear springs with springs from a 3/4 ton truck? What other options are advisable? I've seen "helper" springs that mount to the existing springs that supposedly help. Just from looking at them, I'm a little hesitant to use them.

P.S. - I am not overloading and do not overload the capacity of the truck.