Might want to look at this bulletin, just went throught this at the shop. Subject: Misfire Bump or Multiple Bumps after a 1-2 Upshift
Models: 2004-2007 Buick Rainier
2002-2007 Chevrolet Trailblazer 4WD
2002-2007 GMC Envoy
2002-2005 Oldsmobile Bravada
2005-2007 Saab 9-7X
Equipped with All Wheel Drive RPO NP4 or 4 Wheel Drive RPO NP8 The condition can be perceived as an engine misfire, surge or bump typically during or just after the 1-2 upshift. This condition can occur on Bravadas, 9-7X's and Rainiers NP4 Full time AWD. This will only happen in Trailblazers and Envoys NP8 when AWD (auto mode) is selected. When viewed through the TECH II, misfire counts may be occurring but not enough to set a misfire DTC. The concern will be eliminated in Bravadas and Rainiers with the ATC fuse removed or in Trailblazers and Envoys when 2hi or 4hi is selected. Inspect tire pressures and adjust as necessary. Swap the wheels and tires from another identically built vehicle that does not exhibit the concern. If swapping the wheels and tires does not improve the condition, drain and fill the transfer case with AutoTrac II fluid and re-evaluate the condition.
This is likely a characteristic of the vehicle and some degree of this condition should be expected on all of these vehicles. Comparing this vehicle to an identical vehicle with similar mileage should verify this is a design characteristic. The dealer should have looked at this bulletin and tried this first to see if the problem went away.
Tuesday, April 1st, 2008 AT 8:34 PM