One of my best friends is a GM World Certified mechanic and owns a Duramax himself. He replaced the turbo on his truck recently. According to him, it's not too bad of a job if you remove the cab first! Of course, having access to a lift and the correct tools makes a world of difference. He says he can remove the cab in just a few hours, and always removes the cab for turbo replacements or head gasket replacements.
Casey
1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter