Roadie - thank you very much for your reply. I just made the switch over to GMT Nation (after being referred over here), and I really like the new website. I was back under my vehicle last night and this time I did notice some signs of leakage of TC fluid. I think its coming from around the bearing where the front drive shaft enters the transfer case. When I drained the fluid, I only collected about 1.5 qts instead of the 2 qts that I put in. The fluid was in very good shape and I didn't see any evidence of metal particles. However, I think your hunch that the TC was dry was right on the money.
The seal/bearings around the front drive shaft must have failed and allowed fluid to leak out. I am really bummed because I have changed the TC fluid very religously at even much shorter intervals than 50K. The truck has 170,500 miles on it now and I bet I've change the TC fluid at least a half-dozen times.
I refilled the TC with 1 qt of the GM AutoTrak II and 1 qt of Lucas Oil Stabilizer that says it's good for transfer cases. I figured what the heck, either it will work or not. Was using the Lucas additive in the TC a BIG MISTAKE?
The whining noise did change pitch, but didn't go away. I guess I'll drive for a few more days and see if anything improves. If not, I guess I'll have to shop around for the best price on a reliable shop to fix the transfer case (I sense big bucks going out the door here...)