1996 GMC Suburban C2500

I can offer two suggestions that may provide some clues. First, GM trucks have a natural drone in the drive line, but this can be aggravated by changes in ride height. That changes the drive shaft angle. Raised vehicles have a whole bunch of new problems, but if the truck is at the original height, sagged springs will reduce the drive shaft angle which can intensify the normal drone. You will usually notice a change in the sound and feel when the truck goes up and down over bumps in the road.

For noises with locations that are hard to identify, there is a tool called the "Chassis Ear". It's a set of six microphones, a switch box, and headphones. You clip the microphones to suspect parts, then drive around while switching between them and listening in the head phones. Be aware that many mechanics have never heard of this tool. Suspension and alignment specialists use it to find squeaks and rattles.

Yes, there are inserts made for adjusting the pinion angle with leaf springs, but if you can't find them at an auto parts store, I only know to look at an alignment specialty shop. They will also have the tool for measuring the angle of the drive shaft to the pinion gear.