Toyo upped the Gross Vehicle Weight Rating to 6000lbs. The GVWR is the maximum that the fully loaded car plus the trailer tongue can weigh. The 320lb difference between the Hybrid 4x4 and the ICE 4x4 is a determining factor between max tongue weights.
In the case of the Highlander, my biggest concern would be re: exceeding the rear axle maximum.
After all that, I still *think* you could tow your trailer with a properly setup system. The most I've towed is 2500 and that was on a different vehicle. Get the tongue weight down with the WD and install a brake controller...who knows.

Check out this thread from edmunds http://townhall-talk.edmunds.com/WebX?14@@.f0daea6/1