It's not a terrible job, but has to be done under the truck, and transmission oil will drain out when you disconnect the torque tube ball.
Here's a page from the service manual for Chevy trucks (assuming that GMC's are the same.) Click on the Next Page near the bottom to see the continuation. Note that the figure of the parts for the u-joint has the yokes mislabeled, the one on the right is the rear yoke, and the one on the left is the front yoke that bolts to the transmission output shaft. Rebuild kits are available from the usual vendors.
LINK
