Vibrating between 43 and 54 MPH

My truck is displaying a new symptom. I live close to the main highway, and on a cold start when I pull out onto the highway and go above 40, enough for the overdrive gear, and I let off the gas, I’m getting a loud thud and I can feel it as my RPM’s drop to signify the next gear.

I’ll be getting the tires rotated tomorrow so I’ll have them check the shocks. I’ll have a relative follow me today to see if my tires bounce, and I don’t know what the in the world the thud is.