I tow a caravan with my Pajero. The rear suspension on the Pajero is independent and tends to be soft. The result is with the caravan on the back you get a seasawing affect. My caravan has its own antisway device and you can not fit a wdl device. I have recently fitted air bags. These fit inside the rear coils I fitted them myself it took me about 3hrs. Fairly simple hardest part was jacking the vehicle high enough to remove coil springs. I had quotes of up $1000 to get it done. It cost me around $400 to do it myself with heavy duty airbags. I would recommend fitting them the car rides much better no more seasawing, it rides flatter with less body roll. I inflate them to 20 lbs when towing and deflate them to 5 lbs when driving normally. I could have fitted heavier rear springs but it would mean a harder ride at all times. With the airbags I have the more comfortable ride when not needed. They also reduce tyre wear stopping the rear tyres scrubbing which is a problem with vehicles with independant rear suspensions. 
