Take the half shaft to a workbench and (unless it is obviously broken) examine it carefully along its length for signs of rubbing or 'high spots' recognisable as shiny spots on the metal. Check it with a straight-edged rule for distortion.
Never hammer a slightly distorted shaft in an attempt to straighten it. It can only be straightened in a press. If distortion is severe, a new shaft must be fitted.
Pay particular attention to the splined ends. They must be in perfect condition, with no sign of wear.