If BB can't help with your Duramax, try either Fleece Performance in Brownsburg, Indiana or River City Diesel in East Peoria IL. Fleece did some EFILive tuning for my friend's '05 Duramax/Allison and added close to 100 lb-ft of towing torque and firmed up the shifts in the Allison. And two other Illinois friends have had their "04 Duramax and '08 Powerstroke breathed on at RCD. Both well known in the turbodiesel world. I would have had Fleece reflash my Denali trans to get rid of the shift sloppiness if I hadn't found BB. They know the Duramax trucks really well.
BTW- I'd kill for a Duramax/Allison in my Denali. Tell me that wouldn't be the perfect SUV expecially for towing.