I've lost two differentials on my 190DT. The first was probably the victim of a wheel bearing, which failed a few thousand miles later. The second, a used replacement, may have had prior damage. In both cases, the problem began as a rhythmic drubbing noise. No visible problems with axle, driveshaft or couplings. eventually, the diff failed. The first was relatively gentle...thrubbing, a thunk, then nothing. The second was terrifying...the drubbing sound started a few minutes before the axle locked solid, spinning the car around twice on the interstate. Pieces of shrapnel went right through the back of the case. Lucky nobody was hurt, and the car was undamaged. My current differential has more miles than either of the others...at the first weird sound, the car will be on a flatbed.