I serviced granny's 2011 GLK350 last week, one of her twice-annual oil changes. She told me about a sound she was hearing feom the front of the car when she brakes, so I took it for a spin and heard (and felt) what she was hearing. After I was done with her oil change I poked around a little... Late last year I replaced all of her control arms and related components, and they all checked out. The motor mounts were good, and everything else looked and felt fine - until I grabbed the driver side axle shaft. There is quite a bit of play lateral play in it, between an eighth and a quarter of an inch, and it makes a lot of noise just moving it by hand. There is minimal movement in the passenger side axle.

Here's a video I took: https://youtu.be/wOYJrxBJUnM

This leads me to believe there is a problem with the differential. This combined with the strange, sudden driveline/transmission vibration/shudder that had seemingly disappeared for the last 7 or 8 months but seems to be making a comeback has me concerned that she might be in the market for a rebuilt transmission sooner rather than later... She's owned this car since new, and I have been doing all the service and repairs on it since it came off warranty in 2014. The odometer rolled over 170k last week. I just replaced the intake manifold and throttle body back in March or April, rebuilt the front suspension last December, new tires last fall, new alternator and brakes last summer. Granny is 76 and she really doesn't want to buy a new car. She absolutely loves this GLK, but she's afraid it's going to leave her stranded. She's pretty torn as what to do... if anyone can give me some insight as to what might be going on with the front diff and if I can repair it instead of replacing the transmission, please chime in!

Forgive me, not replacing the transmission per se, but replacing the front differential assembly. Aside from the randomly occurring "rumble strip" sensation (that we cant nail down the cause of), the transmission functions fine.