Brian, You probably do not have a problem. On the Duramax it will not shift into 5th, while in tow/haul, until about 62mph. At speeds lower than 62 it is not in it's effective torque range.
I do not know the perameters set up for the 8.1, but if you look at your tach when it finally does shift to 5th I think you will see it at the bottom end of it's torque curve.
As said above, you can force the shift by turning tow/haul off, letting the shift occur, and then turn it on again.
Happy towing!! Jack
[ 05-20-2002: Message edited by: Jackmchome ]</p>
2007 Classic 3500HD srw 4x4 D-Max, Ally, Reg cab, LB