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?

2 answers
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
why did it fall out and what damage has been done?? need this info before helping you further.