Does it bounce all the time?
I tow about the same weight as you,rear tyre pressures are at 42, I have a McHitch set up and only 14K on the clock, so there is occasional bounce on rougher tarred surface but not alarming and as above you kind of get used to it (its not much bounce).
So I think its "normal", the hitch set up shouldnt make any difference, 140K could be an issue so check that out.

