While making tight turns the transfer case would lock up into AWD and the whole front of the van would shake as the transfer case got bound up. This was due to an incorrect oil called for by GM. It would feel like there was something very seriously wrong with the vehicle the way it shook.

After looking on the internet I found GM service bulletins on the subject and they recommended a transfer case flush and fill. Instead of what was called for originally they now use AC Delco Auto Trak 2 transfer case oil. Which is blue in color. Note: This is for 99 and up only.

After a few driven miles the problem went away and the van now corners just like it should with no shaking in the front end.

My van, having all wheel drive, began having the 'shakes' on tight turns and in parking lots. It was similar to an old 4 wheel drive having the 4 wheel drive engaged and the hubs locked. This vehicle engages this automatically, but was doing it when the fluid got hot from operating at least 15 miles. The fluid was making the transfer case system engage due to sticky fluid. After 8 trips to the dealer for diagnosis, I learned of a service bulletin that suggested the wrong fluid was installed. They swapped the fluid and have never had a problem since.