the rear of my truck is sagging and bouncing while driving
My car has an automatic transmission.
It sounds like you may have some suspension components that may be failing. Often times, when the shock absorbers begin to wear out, they will start to leak the gas or liquid inside of them. When this happens the dampening effect you are used to feeling when driving over bumpy surfaces will be largely reduced. When this happens, you may notice the bouncing or sagging that you describe. I would suggest having a professional from YourMechanic come to your location to diagnose and inspect the suspension on your vehicle.
Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing