Classic signs of 2 potential issues.
1. trailer/hitch/truck set up has trailer nose high. Trailer frame should be parallel with ground (level ground for checking). Having the trailer nose high reduces rear axle travel. Nose doesn't have to be very high to affect suspension travel negatively on a leaf spring suspension.
2. Pin weight - typically good ride has pin weight at or above 20% of actual trailer weight. Mine rides best with ~23% pin.
you don't need helper springs or such if the truck does not sag at the proper pin weight.
Edit: Are your factory overload springs staying fully engaged while towing? if not the symptoms you describe will be present.
