2011 FX2 Eco. I noticed that my truck was vibrating at higher sppeds. I did need new tires. So I purchased 4 brand new tires and changed brake pads and rotors only in front. Vibration continued. I then noticed that it appears that vibration is coming from the back. I can see the corner of my bed slightly going up and down. I am going to replace shocks and brake rotors hoping that will solve the problem. Because any passenger especially in the back seat jiggles lol. Anyone have this issue before? Do you think it is the rear shocks? Another question I tow a roughly 6k lbs travel trailer. What would be a good set of rear shocks? I do not want to put air shocks or adjustable etc. I just want regualr, but I guess heavy duty shocks. Any suggestions?

thanks