Unfortunately, there is no real option to gain more towball weight allowance on a vehicle. I am in the same predicament - I'm over by about 30kg. In my research, there was only one way to allow more weight - an engineering certificate. But even then, thats only the structural weight and not the forces applied to drive train and engine. In the end, i decided to run it as is for now, with a car upgrade hopefully in 2012 if the business can afford it (different vehicles have different ways it is calculated - if I took the winch off the front, I'm legit again, or if I had a petrol variant, I'm way under.....go figure.....) When I spoke to the insurance companies, they did admit they allow a small tolerance, but not in writing of course, and that doesn't always mean it is suitable. Sorry, no help there....
The purpose of the WDH is allow the weight is evenly distributed over the front and rear wheels. If just on the rear, some vehicles may be too light on the front giving poorer steering the braking, and more wear on the rear tyres and suspension. This doesn't affect towball weight as such.
My suggestion is go into a friendly dealer and talk to the service dept. Explain it would be 10kg over and gauge their reaction. No-one will sign off on it to allow it, but they might give an opinion - they know the vehicle better than most.
Also check the max towing allowance - Santa Fe I think is 2000kg max. So with a 17.56-1 at 1657kg, thats only 350kg extra allowed on/in the van - inc all accessories/mods you may order, full water tank, and luggage. You may be pretty close to that tow limit also (??) which would be of a greater emphasis. (the 17.56-1 will be plated to allow 400kg above the factory weight in the van)
Good luck
Andrew
