well i'm sure you've considered this but another variable is the truck tire pressures.
phil in miami tows a long 70s unit with a 1 ton dodge 2x4 with drw...
he is using a air hitch,
Dodge Cummins
Airsafe Hitch
but no can do with the arrow.
he also looked in to the mor-ryde rear shackle rubber suspension mod...
apparently they don't make a product specifically for mopar, but the ford unit is adaptable...
Airsafe Hitch
cheers
2air'