2002 GMC Yukon Differential fluid doesn't seem to be the pr

After driving approximately 20 minutes (usually takes at least this), my GMC Denali begins to make an odd "popping" sound when I turn right or left while in drive or reverse. For example, while circling in a parking garage, turning into a driveway or even a parking spot, etc. If I cut the wheels all the way, it acts as if I have the brakes on instead of just going around in a circle like it should. As the driver, you can hear and feel the "pop". It almost feels as though I am driving on a flat tire or that something is binding. I have been told the sound is not evident from the outside of the car. Something is "popping"/"grinding" and I cannot figure it out. The last visit I made to the auto repair shop was comprised of changing front and rear differential fluids and transfer case fluids (GM Gear Lube and Auto Trak Transfer Fluid), but two days later, the problem came back. This was on 6/22/2009. Any suggestions?