+ Create the locator in the Create menu - and place inside the joint you want the prop to move along with.
+ Display - animation - joint size - ensure that you make the joints smaller so that its easier to see the locator and parent it with.
+ Using parent constraint you can make the prop follow the locator in the joint - joint parented with locator and then locator parented with the prop.
+ Easier to animate the main movement first and then add the prop so that it can follow the main movement.
+ Ensure that the offset is ticked so that the movement will be smoother, however if you want the prop to follow a different joint - create another locator with in the joint and parent it with the prop, however in the channels box ensure that the first locator is turned off or the prop will move between the two locators. For this ensure that the offset is turned off to let the prop transition the movement smoothly - however you can keep the offset but will have to translate the prop into the joint.