Rear end bouncing/Equal-i-zer 4-point 10K hitch

On a washboard freeway it feels like my truck is being shook apart by the TT. When not towing the same freeway is not as bad. I would take the tire PSI up to the max cold and see if it makes a difference just to start. Is that 620 tongue weight a measured or on from the factory info. If you really need to add weight you might be able to fill a holding tank in front just to see if that helps. Then if it does add the weight.