Drive shaft on 2004 GMC Sierra 1500

The drive shaft fell out when my son was driving to work. What is involved in replacing this and how much?

Asked by for the 2004 GMC Sierra 1500
why did it fall out and what damage has been done?? need this info before helping you further.

1 more answer
Usually the rear u-joint breaks and the driveshaft falls out, But like mastertech said "what damage has been done" Tow it to a shop so they can make sure the driveshaft isn't bent and have them replace the broken u-joint and you should be good to go. Here a directory for you