I bought a set of MB rims with snow tires. I wanted to change over my rims and tires. On the back wheels of the ML350, one bolt on each side seems to be stuck. I can move it very slowly with a. 300ft/lb impact wrench then it sticks. All other bolts are easy. Is this some kind of anti theft device? Parking brake problem? I don't want to break my car. This is the 4matic.

Spray and soak the bolt in lots of penetrating oil. And try rotating it in both directions. I had a similar issue with my 2014. I removed all the bolts to rotate the tires. And all the wheels just sat there on the hub! Kicked, hammered, etc....and they just stuck to the hub. Took several attempts of kicking and banging the whee and tires before they finally fell off. My 2014 had spent part of it's life in the Chicago area so I suspect the corrosion seized the wheels to the hub. Now I apply grease to the hub whenever I take the wheels off.

If nothing works then find the next socket smaller and hammer it on with a small sledge. Then use the impact to remove the bolt. The socket and likely the bolt will be sacrificed. Both are replaceable and not expensive.