I have a 2008 ML320, diesel-powered, 4-matic, 130K miles. Recently it started going thump-thump-thump when I accelerated. Turns out that the chain in the transfer case was skipping teeth, if I accelerated too hard. New transfer case was a few thousands dollars from MB. Found a used one (73K miles) online for $700. Re-manufactured ones are available for $1K - $2K. (Search for re-man.) My old one was low on oil, and the oil was brown; should be red. No information anywhere, that it should be changed, or what kind of fluid to use. The independent garage who swapped it out for me ($375), knew what fluid to use. The 'new' one's fluid was also brown; changed it. Took the old one apart. The chain was stretched - very loose. No sign of wear on the chain, or the two sprockets. (Chain and sprockets are about 1.5 inches wide - look pretty strong.) Motorcycle chains and sprockets "stretch" due to abrasion, by particles trapped in the oil, so it's actually 'wear', not 'stretch'. In this case, I see no signs of abrasion, and I conclude that the chain links actually "stretched" under load, which will occur if the steel gets heated too hot, and/or the load is too high. Low, brown oil indicate that's what happened. The chain got over-heated.
SO: if your vehicle has a 4-matic, and particularly if you tow a trailer, or travel long distances at high speeds in hot climates, CHECK AND REFILL YOUR TRANSFER CASE, about every 20K miles. The MB service manuals don't mention it, but it's YOUR RISK if you don't. Doesn't take long to drain-and-fill, and only takes about a quart of fluid; shouldn't be too expensive.
Any experts out there who read this article, please inform us what kind of fluid to use, and who sells it.