Have replaced 7-10 transfer cases & repaired several others, but never even heard of a bad front diff.
Most common problem is the clutch pack in the transfer case going bad & allowing hydraulic fluid into the transfer case.
Iwould check the transfer case fluid level & also see if the 4-matic fluid level is going down.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)