I had this happen to my truck several years ago. I my search for a new pulley, I learned about a reinforcement washer that GM put under the pulleys to help stop cracks from developing. I put one on with the replacement pulley and have had no problems since. I'm not sure what year GMC started doing this but I have seen them on just about every year of truck over the years of junkyard explorations. They could have been added aftermarket by the dealer also.
It would be easy to make one. It's just a disc of flat steel with the edge beveled to nest inside the pulley's inside radius. I think they were about 3/16" thick.

Hope this helps.

"The Operation of GMC trucks is normally smooth and steady, with no loud rattling, knocking or unusual noises. A good driver will quickly become accustomed to driving a GMC truck and will get the feel of his vehicle regardless of his engineering knowledge."-TM10-1563