A
link to a WDH explanation.
But, a different way to measure, just to see how much squat:
Have TM level, on the levelers. Measure to the bottom of the hitch, where the ball will go. Write it down. Back your Highlander close to the TM hitch. Measure to the bottom of the ball mount on your drawbar (right where your ball is mounted). Write it down.
Hitch the TM to the Highlander. Make sure no stabilizers are down, just as you are when you are ready to hit the road.
Measure to the bottom of the TM hitch, the same spot you measured to before. Write the measurement down. Measure to the bottom of the drawbar, where your ball is mounted. Write it down.
Compare the measurements to one another. Is your TM hitch at the same level as when unhitched, but level? How much difference? Is your ball mount the same level as it was without the trailer attached, and unsupported by the leveling jacks? How much difference?
My guess is that your ball is not the same level as it was without the TM attached. If this is the case, your drawbar is lower. If it is lower, then your headlights are aiming higher. If you are driving at night, your low beams will appear to be high beams to oncoming traffic, and they will flash you. That is but one reason to get a WDH.
Other reasons, as mentioned: Not enough weight on the front wheels, which impairs steering and braking. Too much weight on your rear axle. Highlander is not riding level, causing all these problems.
In my mind, unless someone has a very stiff rear suspension, towing anything other than a utility trailer will require a WDH. I did not use a WDH when we had our 1,200 lb popup. I didn't drive at night, but I bet my lights aimed high. I do know my rear sagged (and not just because I am aging!), but didn't think about it. Since I got the TM and started reading the forums, I have learned much about proper towing setup and equipment. Glad I got onto the forums!
Good luck, and keep us informed of your measurements.