99 2500 possible transfer case issues
 

I have a 1999 Silverado 2500
6.0 L engine
3.73 gears
153,000 miles

Recently while towing my 4500 lb trailer, I began to hear a noise coming and going while going up a few small grades at approx 70 mph. I then let off the throttle, selected 4 wheel high and everything sounded ok. I then went back to 2 wheel high and after a few miles, the sound came back. I was then able to go into 4 high, then 2 high and again, the sound stayed away for approx 40 miles, then it came back.
I just drove the truck without pulling the trailer and it happened again. When the noise is present, the truck feels like it has some mechanical drag, much like when in 4 wheel high.

Could this be a problem with the transfer case shift motor?
I have not drained the transfer case yet.

sounds like its possible that the shift motor is acting up but its hard to say with out draining the case and seeing what you find...........I would take it to a shop and run the codes and do a live scan while it is being driven (takes a special scanner) this will shed lots of light on the issue