That combination will be great. The 3.42 is the lowest gear ration offered with the new 6 speed and it will pull 4,000# fine. We used to pull a 6,000# scaled wieght TT with an 05 Tahoe similiar equiped except it has the 4 speed auto. Towed several tens of thousands of miles (like 50k) with it and no problems at all.
As far as reliability goes that 05 Tahoe is still on the road with DD. Has 160k miles on it and it still runs like new (actually better than new off the lot) and it has never been touched mechanically except for all required maintence in accordance with the heavy duty schedule
We like that Tahoe so much we have an 11 Tahoe as well. Great vehicles