I couldn't really tell you much about the MLs 4wd system, I haven't experienced it first hand. Although, being a competent offroader is more than just a slick/advanced 4wd system; wheelbase, front/rear overhang, ground clearance, weight, suspension, chassis design, tires, etc. all play even more of a role.
To my understanding there is an offroad kit (for the ML) that has been available as well as a centerlocking diff that will be available if it isn't already.