hello!
There are a couple possible causes for what you describe:
(1) It is absolutely essential that all 4 tires on this type of vehicle are of the exact same diameter. If you have a tire that is low, is worn differently, is newer than the others, or if you have mismatched tires (even if they have the same size printed on them), it will cause slippage in the transfer case clutch pack and cause transfer case damage. Tires on this type of vehicle always have to be replaced in matched sets of 4 at the same time to prevent transfer case damage (check your owner's manual).
(2) There is a service bulletin from GM (number 99-04-21005E) which describes a condition very similiar to what you are experiencing, caused by worn out transfer case lubricant. There is a procedure for treplacng the transfer case lubricant to try to correct this condition; if it does not work, the transfer case will need to be disassembled and repaired. you should also be aware that the transfer case in this vehicle requires a special GM lubricant; adding anything else such as Dexron transmission fluid to it wwill cause chatter issues and possible internal damage.
The 4WD system on this model also has self diagnostic capability, but you will need a Vetronix tech-2 scan tool to be able to access the vehicle's data stream to be able to diagnose control problems or internal faults.
I would recommend taking this particular problem in to your local GM dealer service department; they do have the required equipment and technicians trained in this system to be able to diagnose it for you accurately. Some independent shops do as well, but finding one can be difficult...
I hope this is helpful; if so, please click mya ccept button! Thanks!
