Mine is a Championship 2009 that the dealer used for 1000 miles as a "demo" and I know took it off road once. It made that noise when I test drove it. They said they found a bushing split in the suspension, I suspect it was the one you are targeting here.

 

khoolhandz, Iv'e got the exact same issue -first and second seems to be a little rough of a shift, though my 09 Alpha T has only about 10K miles on it.

Hummer Guy, any news on the part number?