The early ML320 and ML350 were definitely ladder-on-frame, and with hi-lo transfer case (1st gen W163). Up through 2005 model year. I’ve read that and seen the frame members myself. 2006+ was unibody (just confirmed). In fact, I started thinking the other day about one of them to replace my own rarely-used tow dedicated SUV, based on what I’ve learned. The lack of a transfer case in it also makes it not a “real” SUV in my usage and in many definitions as there are things I expect to be able to do with an SUV that do require a hi-low transfer case (getting to certain mountain campsites, etc.).
My only unibody SUV towing experience was with a Jeep Grand Cherokee. It was a disaster (two-horse trailer through North Cascade mountains). In fact the owners offered to GIVE us the vehicle (family members). We politely declined and went and bought a Toyota Land Cruiser instead. I will grant the 2nd Gen ML may be up to the task with some more significant engineering; I haven’t tried that but my first reaction was neutral on that point given my (clearly limited) unibody towing experience. Either way, this is as a grocery getter/dog hauler/etc. No explicit need for towing. However if it were capable, I could get rid of my seldom-used SUV and borrow this one when needed, so that’s a potential plus for me in all this.