One additional item for them to look at is the engine mounts as they are a somewhat known problem as the isolators in mounts get brittle and fall out as a result of heat damage from being so close to the exhaust manifolds on the Gen III engines. This can cause an audible popping noise and a horrible sensation through the steering wheel similar to a dry intermediate shaft depending which mount fails. In most cases, you will not notice an engine vibration although you should see the air intake pipe will be very close to the fan shroud, possibly to the point where the two start wearing into each other.
About the only way to get a good view of the mounts to verify if the mounts are wasted is to remove the front tires and inner fenders and peek at it from the side.
GM released a new engine mount with a heat shield to prevent this from reoccurring. The new design mount is 25828247.
Hopefully this will help you as I have seen many of these trucks with this condition. Good luck and let us know what they find.
Here is the new mount vs. a toasted original.
