Truck Repair: gmc sierra drive shaft, gmc sierra, sierra drive

Question
Hello, I drive a 1999 GMC Sierra 1500 Z71.
Recently I drove from Edmonton to California and was pulling a trailer.  When I reached a very warm Las Vegas, I noticed a loud tinkering coming from the mid section of my truck.  I did not get this looked at.  A few days later the tinkering was gone and I have vibration when I travel above 55mph.  I don't know if these two things are related but do you know what could be causing the vibration?

The auto shop thinks that it is the driveaxle.  But they are not sure.   

Thank you

Answer
Hi Jeff,
Just a "U" joint in the driveshaft could cause that. Set the parking brake, put the transmission in neutral, and slide under there and twist, pull, push, and wiggle the driveshaft to see if there is any slack.
If you have the rear axle jacked off the ground, and the parking brake off, trans in neutral, you should be able to turn that shaft by hand, and see if you can find slack.
But I am betting on the "U"joint, rather than a drive axle, although that possibility exists.
Van