These couple of articles are always worth reading when you are towing something that is getting up near the car's maximum towing limit.
http://caravanandmotorhomebooks.com/caravan-dynamics/http://caravanbuyersguide.com.au/tow-vehicle-caravan-weight/Don't forget the advertised maximum towing and carrying weights are for good sealed roads only. They should be reduced accordingly as road conditions deteriorate.
The following is an email from Mitsubishi. If you contact other manufacturers they will most likely tell you the same thing.
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We acknowledge receipt and thank you for your recent enquiry regarding the Mitsubishi Triton.
Please be advised that Mitsubishi Motors Australia Ltd (MMAL) only release towing
specifications for vehicles being used under normal conditions such as highway driving. We would not recommend towing this amount while driving on 4WD tracks or surfaces of this nature. The 3 Tonne towing capacity is a statement of the maximum permissible towing weight possible for this vehicle.
Thank you again for your enquiry.
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Another issue is your car manufacturer's towing instructions in the owner's handbook. They might say a weight distribution hitch must be used for ball weights over a certain limit. Unfortunately a WDH is not suitable in many off road conditions. Do not ignore this if it is in your book. To do so could easily lead to broken wheel studs, a cracked or bent chassis or a broken rear axle housing.
A WDH levers the rear end of the car up and takes weight off the rear axle. That removed weight is then distributed between the front wheels of the car and the wheels of your trailer. Heavier springs or air bags lift the chassis but they don't take any weight off the axle or redistribute it anywhere
Another point is rear axle load. All tow balls are well back behind the rear axle. The distance from the ball to the axle is a lever. The weight a trailer puts onto the rear axle will be a lot higher than what is on the ball because of that leverage effect. When you put the car into motion, the axle load will increase considerably as the car bounces up and down and can easily exceed the axle's maximum load limit when combined with the weight that is in the back of the car.
That is one of the reasons why towing and carrying weights must be reduced in rough conditions.