When it came to driving the steep grades in the Sierras I used the ML's unique ability to employ low range on pavement ( a feature not available in most other 4x4 ). Even with 4100 lbs in tow it felt like storming up the steep grades.
The ML320 consumed significantly less gas on the freeway as well as on steep hills.
Later I decided to employ low range for towing in town as well. What a difference! Using low range for towing heavy loads around town results in a completely new "tow-feeling". It feels like you forgot to hook up the trailer - it feels like nothing is holding you back or slowing you down anymore.
As long as you don't intend to go faster than about 50 mph you can use low range for anything you have in mind - no limitations like with other SUVs. (Engine speed @ 50 mph in low range 5th gear : about 5,000 rpm)
One of the revolutionary features of the Mercedes-Benz M-Class is, that low range can be used on all surfaces - that means for towing of heavy loads it can be used on pavement or other surfaces with high traction.
Traditionally all other 4WD vehicles (neither with part time 4WD systems nor with full time 4WD systems) can't use their low range setting on pavement.
They can't use low range on pavement, because
a) part time systems do not have the necessary center differential. Accidental use of low range would lead to damage of driveline components and make driving unsafe due to heavy understeer and ABS no longer working properly,
b) full time systems do have the essential (open) center differential for safe steering, but in most full time systems the center differential is automatically disabled (locked) when low range is engaged and all the negatives effects known from part time are displayed.
The reason why I chose low range for towing up steep grades and around town, was that the ML's crawl ratio in low range provides 2.64 times more torque than in high range. Needless to say that towing becomes much much easier. I noticed also that the coolant temperature did not rise as much in low as in high range.
A very welcome side effect is that the automatic transmission generates significantly less heat due to reduced "slip" of the torque converter - resulting in longer transmission life.