Weak springs will allow the axle to hop all over tarnation instead of resisting the hill or valley that the wheel goes over.
That's really a function of failed shock absorbers. Weak (sagged) springs will just change the height of the suspension. They'll likely have the same amount of spring rate (springiness) although seized bushings can make them stiffer (unable to move like designed.)
If you do decide to replace your springs and bushings, etc, I would do both front and rear to keep the truck level and prevent the front end from doing one thing while the rear end does something else on the same bump in the road.
Spring rates on front and rear were different as the came from the factory, so the front and rear will always to different things on the same bump. But changing from the factory spring rates on one axle would make the truck act differently. But again, shock absorbers control the bounce or hop.
Straight front axles are still used today (many 4x4 trucks), and they ride satisfactorily.
