Trying to find exactly where the sensor is that measures front tire speed 2011 F-350.

After recently getting stuck in some awesome Texas gumbo mud and having a "Check 4x4" light come on I'm trying to see if the sensor is getting mudded up?

I looked all over the hub but couldn't find anything that looked like a sensor.

Any help appreciated.

 

With earlier models it was on the transfer case, but I heard that later models use the wheel ABS sensors. I have not confirmed this yet.