Automatic Transmission Column Shift problems of the 1998 Oldsmobile Bravada

This vehicle shudders on dry pavement. There is a problem with the all wheel drive system. Please see bulletin # 99-04-21-005 (this is on your website) we have had the vehicle in for service 4 times for this problem. They changed the fluid in the transfer case 3 different times, but the problem still persists. We have contacted Oldsmobile, with no results. Can you please help us, we don't know what to do next. The vehicle is still under factory warranty. Gm/ Oldsmobile refuses to replace the transfer case and the mechanic at pence Oldsmobile suggested this is what should be done to correct the problem.