It's Defenately not an engine limitation.
The same 3.6 pulls over 5000lbs on the Durango, and the grand caravan pulls 3500lbs w/ same engine and tranny.
The engine bay isn't over crowded as compared to the caravan, so I don't think it's a heat issue either.
It's got to be chassis limitation. Maybe the suspension can't handle it.
On another note, is the Durango available in NZ? I'd look at that. It's avail w/3 row, tows 5000+ lbs, and has the same v6 standard.
