99 Z71 Silverado 4x4 stopped working.

I check the 4x4 every once in a while during the summer just to make sure it works. I had the drivers side axle seal replaced just recently.
The motor "sounds" like its engaging 4x4.
I've already check the fuse in the panel next to the drivers door.

Sensor?
Module?
Plug?

Thanks for the replies!