4x4 engaging issues

Good morning Members: Im new to the forum so Im hoping to be posting in the correct thread. I have acquired a 2012 XUV 550 s4 that has started to have issues with engaging the four wheel drive. The machine works fine in two wheel drive but when I engage the lever it acts as if it won't fully engage. The rear diff grinds and pops until I take it back out and run the machine forward and backwards a few times, and that takes some work to even get it to move. Before I start tinkering can anyone give me some pointers on where to start so Im not chasing my tail.

Thanks in advance.

Andy