One problem related to bumper has been reported for the 2001 GMC Sierra. The most recently reported issues are listed below.
Backing up at about 2mph hit a pipe sticking out of the ground. The pipe was about 3 ft. High. It hit the rear bumper on the driver's side. The bumper was bent so badly that it had to be replaced for a cost of almost $600. 00. I think this bumper should have withstood this type of accident.