The Envoy/Trailblazer/Ascendor uses the 4L60 GM tranny instead of the 4L30 one that the Isuzus currently use. The 4L60 can handle up to 350ft-lbs of torque instead of the max of 250ft-lbs that the 4L30 can take. That is why they had to go with the other Tranny. As for the TOD, it's probably a result of the TOD costing more $ from B&W cause they would need a beefier TOD unit to handle the additional torque. Most GM buyers wouldn't care either way Auto-4wd v. TOD to them it's all the same thing, and would cost GM more $ to produce. Remember now that it's a mass-market vehicle, every penny they save is multiplied by a ton of vehicles which translates to a lot of $ in the end.
Links for the Tranny's below: