Just an idea I had considered...why not support the bed of the trailer with a series of short rubber or bungee straps? The straps could attach from the frame or chassis of the trailer to the bed along several points and provide a uniform vibration damping system. Granted, the straps would have to be short to allow for space considerations between the bed and the frame, but it might work, if you have enough of them and if they are of the proper elasticity.