[inghamm]

I think Bruce is right. TrailManors are renowned for NOT swaying, but part of the reason is more of the weight is forward of the TM wheels, rather than after. If it was 20 gallons of water on a rear hitch, you put 167 lbs WAY back with lots of leverage. What if there was room for the liquid to swish around back there also?

I think my rear hitch is rated at 100 lbs. I put a couple of bikes back there and also use the Andersen no-sway WDH.

One other thing, could be the opposite - too much weight on the tongue bringing the Highlander's front wheels up and not getting good grip on the road. Have you checked the measurements of front wheel wells to ground unloaded vs loaded and hitched?