What can I do?

it sounds like the trailer motion is being transmitted to the truck. Have you checked/replaced the trailer shocks?? I'd also check the rest of the trailer suspension to make sure it's in good working order, greased/lubricated where possible, equalizer (if equipped) functioning, etc. Also, do you have a standard pin box??? If so, may want to think about switching to one that has some shock/motion absorbing feature like TrailAir, MorRyde, etc. Another thought is to check the hitch itself to see if for some reason there's too much play where it engages the trailer pin....just trying to think of things that may be giving you the motion...