2002 Cadillac Escalade EXT Noise/Vibration

2002-2005 K-trucks were pretty common for having an issue in the front axle where the left side bearing adjuster locking tab would break. The adjuster would eventually move altering front axle backlash and preload setting. You can pull the front diff drain plug and check the magnet for metal. A little is normal, excessive amount is indication of a problem. Through the years, I have seen as little as scored bearings, to a couple occasions where the axle had seized. This is just food for thought, and would relate more to a humming/growling noise, more than a banging or thumping. It all depends on the severity. You say the transfer case fluid has been silver for many years, was this known and never addressed by servicing, or has it been serviced and returns to a metallic state? Is there any kind of hesitation, or feeling when this occurs?

If you can duplicate it with a torque brake, the bearing adjuster would not be a possibility. Do you have an issue with the front axle, it is highly probable. If you can duplicate it stationary, then a second person either running the truck, or determining where the noise originates from, would be the best option. Nothing comes to mind on what the cause could be, without being able to hear it.