I don't have any CS recommendations but when the transmission went out on my 04 Suburban I had it done by High Country Transmissions here in Denver.
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Assuming the Trailblazer has the same 4L60E transmission as most half-ton GM trucks, repair/rebuild is pretty common. My understanding is that the transmission was built with some weak components at the factory that can break or go bad. The good news is that most shops rebuild with better components so (in theory at least) once the rebuild is done the transmission should last the life of the vehicle.
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My rebuild was exactly $2,000. I wouldn't pay much more than that unless he wants to upgrade more of the components.