Clicking sound when in gear, why?

This sounds like the driveshaft U joints need to be replaced, but to be sure get a flashlight and inspect the U joints for rust colored dust at the joint this is a sure sign the U joint is worn out. Can you please shoot a quick video with your phone so I can hear the noise? That way I can tell for sure what's going on. You can upload it here with your response. You can remove the driveshaft. Here is a guide to help and I have left instructions in the images below on how to remove the driveshaft for inspection:

https://www.2carpros.com/articles/comprehensive-guide-to-checking-and-replacing-an-automotive-drive-shaft-u-joint

and

https://www.2carpros.com/articles/how-to-remove-a-drive-shaft

Check out the images (below). Let us know what happens and please upload pictures or videos of the problem so we can see what's going on.