I'm sure i read somewhere that they have special rear suspension to deal with the towing they do, so you cant change the springs. Or is that a load of twaddle?
If its an ex AA spec lift?Then the rear suspension is air operated to let them lift all different sizes of weight and still be "road legal".Hope this helps
Give Your Chassis Number To Your Local Vw Commercial Dealer And They Should Be Able To See Whats On There On A System Called Etka. Buy Oil & Filters To Keep Em Happy.