the condition is almost common, and GM has introduced a service bulletin to address this concern, a copy of it is below.
the case and concern are listed below too
This condition may be caused by a slip-stick of the clutch plates in the transfer case clutch pack. On 1999 to 2002 models, the preload that is designed into the clutch pack may contribute to the condition. In addition, contamination of the fluid may interfere with the friction modifiers.
Flush and replace the transfer case fluid on affected vehicles using the following procedure and the part number listed below.
Perform the following diagnosis prior to flushing and replacing the transfer case fluid in order to properly identify the shudder
Bulletin No.: 99-04-21-005E
Date: November 28, 2005
Subject:Transfer Case Shudder (Flush and Replace Transfer Case Fluid)
Models:1999-2005 Chevrolet Astro (AWD)1999-2005 GMC Safari (AWD)1999-2001 Oldsmobile Bravada
with All-Wheel Drive (NV 136) Transfer Case (RPO NP4)
This bulletin is being revised to update the correction information. Please discard Corporate Bulletin Number 99-04-21-005D (Section 04 - Driveline/Axle).
Some customers may comment on a vibration (shudder) during tight turning maneuvers on dry pavement at vehicle speeds less than 8 km/h (5 mph).
Perform the following diagnosis prior to flushing and replacing the transfer case fluid in order to properly identify the shudder.
Important: Refer to Corporate Bulletin Number 99-04-21-001 dated April 1999, for important information on removing the transfer case drain and fill plugs.