Its the joint that loosens, there will always be an element of twist at the transfer box.
Grab the prop close to the joints and push and pull them to see if there is any joint movement, up down all ways.
Also have a close look at the internals of the joint area, if you can see any shiny bits or even needles then the joint is fubar.
Just because a joint is fubar does not mean a new prop though! These joints are only about £6 and take 20 mins to change (plus the time to remove the prop)