This happened to my 99 ML430 at 50K mi:

Squeeled suddenly and loudly as I pulled into garage, engine vibrated violently. Smelled burned rubber.

Called shop next day, and they knew right away it was the balancer. I started the engine and it ran smoothly. Drove 4 miles to shop.

They found that the balancer was indeed bad, but that it also ate through the oil pan which had to be replaced too. Total cost parts and labor was around $950. (10 hour job !).

The shop foreman said the balancers have improved in design over the past couple of years, and the separation shouldn't happen again. Hope he's right !