Loose unsecure hard heavy plastic items inside the 2 most rear cubbie pockets, and or the doors to those pokcets
Emptied those compartments along with the cover doors, threw them in the cab on passenger floor, then went at it over some of the worst bumps in the area I was in and no more banging!! back there like it had been
Didn't even know those trucks had any bed storage cubbies at that time. All mine had were some storage area under the extra cab seats.
My 2020 tacoma came with a bed storage compartment, but I haven't use it yet.
Not all kyb shocks are created equal. They have different models. Idk, if you can even buy the original kyb shocks that came on Toyotas at the time, other than thorough toyota. I think the aftermarket ones are different, and don't perform the same.