Airbags are NOT designed to level your tug/van combo. You use a WDH. Don't confuse the two and it is important in terms of safety that you understand the concept of a WDH. I have air bags in the back of the Landcruiser to help the suspension with the extra weight. I also have the option of fitting harder springs, but this would make the ride harsher when not loaded. With the bags you can reduce the pressure and go back to a supple ride when not towing.
Got mine online from a US site at better than half the cost here and delivered to my door within 6 days.
Larry

Lots of Territory owners fit airbags to level the rear either with or without a WDH. I use a 4 bar WDH yet is still does not level the car so I will probably buy airbags as well.
I've attached a link to a Territory forum article on airbags.
There is also a dealer called "Airbag Man" who sells them for this purpose:

You can adjust for varying loads & road conditions by simply inflating or deflating, just like inflating/deflating a tyre for front-to-rear & side-to-side levelling control, this enables a vehicle to "level up" when towing caravans, boats and trailers or when a vehicle is carrying heavy, uneven loads.

http://www.fordforums.com.au/showthread.php?t=11429217&highlight=airbags

The coil rear end sags on Territories and most coil sprung vehicles. Dollar for dollar airbags I reckon are the go with the addition of a WDH if the van is over about 1400KG ATM.   Cheapest way to buy a WDH is Gumtree/Ebay where they are commonly available 2nd hand.

Good Luck