Romer said:The CDL does not put the Tcase into LO. It locks the front and rear axles for better traction while you are in high. Otherwise, the only way to lock the front and rear axles is when you are in Low.
So in snowy weather, you can drive at 40MPH with the axles locked.
This is different than the lockers in the axle which lock the tires to each other. See the diif for dummies link in the FAQ
Actually I think CDL just locks up the center diff or t-case forcing equal amounts of torque to the front and rear diffs.