Ben says to tow in 3 instead of D for the same reason you towed in 3 with your old rig, because the GMT400 manual tells you to. For the GMT800 it says tow in D with tow mode on.
Even though it is the same transmission, the software has changed significantly between the GMT400 Vortec and GMT800 LS era. The LS powertrain computer is something like 100,000 times more powerful than the GMT400 computer was. Both owner's manuals are absolutely correct the trucks are just slightly different. Just do what the manual says and you should be OK. Hopefully you had a breakthrough here, but unfortunately I think the 80 still locks up in third when in 3 (direct.) I hope I'm wrong and your right.
