I wouldn't expect metal shavings. We are talking about a motor, not a transmission or rear end. If there were any metal, it would probably be in the filter between the pleats anyway. In fact it probably wouldn't hurt to cut the filter cartridge open and take a look. Dropping the pan on a GMC is a relative simple matter. Check the gasket on the rear main to make sure it didn't slip. It could be the that is all that is wrong. Make sure the float is still on the oil pump and see that the set screw is holding the pump whee it belongs. I would remove all of the spark plugs so you have no compression and let someone turn the engine over while you are under it and look and listen for something out of the ordinary.
If the rear main was was indeed leaking, you need to pull the rear main cap to change the seal. The top shell should still be in the block and the bottom shell should still be in the cap. Describe what the bearing looks like. I would also pull the number one rod bearing cap and take a look at it and let us know what it looks like. If nothing looks bad like a spun bearing (you will know it if you see one) I would check bearing clearances with Plastigauge.
http://www.plastigaugeusa.com/how.html


I think most auto parts houses still have it. Read the instructions closely before you ever start.

If the rear main and the number one rod are within tolerance, you might as well Plastigauge all of them. I would recommend one of the Sneaky Petes to install the rear main seal.

http://www.amazon.com/Lisle-27000-Sneaky-Pete-Tool/dp/B0002SRCJW

Do you have a shop manual that shows you how to reinstall the rear main seal? It will clearly explain how to pack the seal and trim the threads.

There really isn't a step by step procedure for trouble shooting what you describe. However, I always look at the simple things first. As an analogy, an acquaintance of mine was recently worrying about having to take chemo because he just knew he had melanomia. Turned out it was merely an ingrown hair.