Since the weight of the ML is just a little bit more than the S(talkin about the 320s), why does the truck need wheels with much higher load capcity than the S? Is it because of the "towing" ability that the ML has? If I do not use the ML to tow at all, is it possible for me to use lower load capacity rims?

no one got any answers for me?

I haven't looked at the charts yet, but ApeG, I believe the maximum gross load capacity of the ML is much higher than that of the S. That must have something to do with it. Also, the load distribution on the tires for the all whell drive truck is very different from the cushy ride suspension geometry of the S class. Thus, though both vehicles weight approximately the same, the ML rims is designed to accomodate a vastly larger range of load distribution, even under normal driving.