($)4-matic is only a traction system. What you are describing is an automatic transmission problem OR design "flaw". The 4-matic system works upto 12 MPH every time that you start from a standstill. Other than than that it will work IF & when wheel slippage is too high below 25 MPH.
Sounds like the transmission modulator is out of adjustment OR defective. Have you had the ATF in the transmission & in the TRANSFER case changed? They have their own fluid capacities.
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