Taking things in opposite order, you don't need to replace the receiver that is on your vehicle. A WDH has a 2-inch bar that slides into the existing receiver. On the end of the bar you'll find the ball mount (the place where you bolt on the hitch ball) and the springbar sockets - the heart of the WDH.
Do you need one? There are two real ways to tell, and if you can, you should check both. First, measure the height of the tow vehicle at the front bumper, and at the back bumper. Then put the expected load in the back of the tow vehicle, put the tongue weight on the hitch, and see a) how much the back end of the tow vehicle squats, and b) how much the front end of the tow vehicle rises. If both the squat and the rise are small (maybe 1/2") then you probably don't need a WDH.
The other way is weigh the front axle of the tow vehicle, then load the vehicle up with the cargo weight, put the trailer's tongue weight on the hitch, and weigh the front end again. The front end will now be lighter than it was; if it is lighter by more than about 100 pounds (round number), then you need a WDH. Ideally, the weight on the front end should increase, since the front end now has to steer and stop a bigger load. But without a WDH, the opposite happens - it gets lighter. A WDH corrects this problem.
The second method is more complex, since it requires access to a scale. So most people use the first method alone.
These numbers are guidelines, of course, not drop-dead hard requirements. To summarize the expectations, most "ordinary" vehicles need a WDH, the exceptions being larger SUVs (on the order of a Ford Expedition, but not an Explorer) or a heavy pickup truck (perhaps a Ford F-250 but not an F-150), or a vehicle with a very long wheelbase, such as a full crew-cab pickup.
To be more specific, an '08 Highlander must have a factory-installed towing package in order to tow a TM at all. And even with the tow package, it almost certainly requires a WDH.
Bill
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