Whining noise apparent from either transfer case or front en

I have a 2000 GMC Safari AWD, 4.3L with 98,000mi. Back in Dec2006 I had the transmission rebuilt due to the "sun"gear failing. Over the last 2 weeks I have noticed a whining noise almost like a gear noise that has gotten quite audible. The noise is not grinding like a bearing failure but more like a gear mesh or belt kind of noise. It increases and decreases with the speed of the vehicle. No change when in nuetral and coasting. Checked the front and rear differential fluids and added a slight amount of gear lube to the front. No change. What should I be looking for? 8)