have a look at the rear wheel speed sensors tim, juck the van up and use a multimeter on ac volts and then disconnect the sensors multiplug connect the meter ( im assuming there std abs inductive sensors here ) you should then spin the wheel and check the sensors output in AC volts, will vary with speed of rotation. the merc from memory utilises the rear abs sensors as part of a basic traction control system for rear wheel spin under acceleration, if one sensor is out of limits it may see it as wheel slip
