If memory serves correctly:
E350/E450 chassis 2004 and below had O/D button (overdrive lockout)
E350/E450 chassis 2005 and above has Tow/Haul and the five speed transmission
As previously stated Tow/Haul on the Ford changes shift points, prevents gear hunting, and on down grades if you tap the brake it will automatically downshift to a lower gear to aid in braking. Some times I find it a little too aggressive on the downshifting.