I bet you do need a WDH. MOST (but not all) class III hitches have a 500 pound tongue weight limit without a WDH. MOST trailmanors get close to or exceed this limit.
Most of the factory tongue weights are dry weights. No battery, no cylinders of LP, no awning, no water in tanks, no pots and pans, etc. etc. The only way to tell is to weigh the unit. Most accurate way is at the scales on your way out to go camping at a truck scale. Second best is in your drive, fully loaded and ready to go using a bathroom type scale. Here is one of several links on how to do this
http://www.ehow.com/how_5846230_figu...er-tongue.html
If you are like most people you will be amazed how heavy your tongue really is!
On another note I think the 4Runner is built on a frame like a truck where the Highlander is unibody. Unibody can flex and sometimes will be harder to control at higher speeds due to its construction and lighter construction. I am sure there are highlanders out there that will say I am all wet. I do worry bout exceeding the spec limit of tongue weight but could not find what it is on the net. ASK the dealer specifics before you buy!
Dave
Folks can tow without the WDH with no noticeable problems but are damaging suspension, bearings etc. of the rear end and stressing the connection of the hitch to car beyond its weight limits--it is a problem (read disaster) waiting to happen. Is it worth the $200???? Only you can answer that question.