I own a 2006 Highlander AWD V6.
A few points:
-There is absolutely no mention in my owners manual about
weight distribution or equilizing hitches. Here's a copy & paste of what my manual says, you can look it up on Toyota's website in the manuals section:
HITCHES
If you wish to install a trailer hitch, you
should consult with your Toyota dealer.
Use only a hitch recommended by the
hitch manufacturer and the one which
conforms to the total trailer weight
requirement.
The hitch must be bolted securely to
the vehicle frame and installed according
to the hitch manufacturer’s instructions.
The hitch ball and king pin should have
a
light coat of grease.
Toyota recommends removing the trailer
hitch whenever you are not towing
a trailer to reduce the possibility of
additional damage caused by the hitch
if your vehicle is struck from behind.
After removing the hitch, seal any
mounting holes in the vehicle body to
prevent entry of pollutants such as exhaust
fumes, dirt, water, etc.
That's all there is.
-I don't own a FG trailer (yet). But I've been towing a 2500 lbs Coleman popup (about 300-325 lbs tongue weight) with the HL for some time, and I use a WDH, Reese's now-discontinued 350 Mini. Absolutely no issues, and honestly I wouldn't tow without it. The rear suspension of the HL is quite soft, and the WDH levels the HL and improves the ride and handling drastically.
-All Cequent-made Class III hitches made for the HL are not rated for WD. This means Reese, Draw-Tite and Hidden-Hitch. When I bought my HL (used) it came with a Cequent hitch. I replaced it with a Curt, which is WD-rated. Also the Curt doesn't extend as far back as the Cequent, which is good, it looks better and the finish and welds seem of higher quality. And yes, the Curt also has the loop that goes under the exhaust pipe.
So Cequent prohibits WDH on the HL, not Toyota.
-The OEM and aftermarket hitches all bolts with the exact same 6 bolts under the vehicle. No difference. Most Class III hitches bolt the same way, most are WD-rated. No issue here. Undibody flexing is nonsense if you stay within the vehicle's limits. The OEM hitch shown on page 4 of this thread is for a '08 HL, which is the 2nd generation of HL and an entirely different vehicle (with a 5000lbs towing capacity) from the 1st generation ('01 to '07) so no wonder why this hitch is different from the Curt just below it, it's made for a different vehicle...
If it's rated for it, it's rated for it. Doesn't matter if it's unibody, if there's a loop under or over the tailpipe, it's Camry-based, this or that, I've heard them all. And if there's no mention against something, it's not prohibited, then stay within the limits and go for it.
Carl