When in tow/haul mode all drive modes are selectable, i.e. 2H, 4A, 4H, etc... For normal highway towing is there a reason to use 4A vs. just 2H. Gas efficiency, control ??? I towed in both modes and didn't tell much of a difference.
If 4wheel is needed I would use 4A for towing on snow/ice covered asphault or very hard surfaced roads as it still allows front /rear driveline differentiation and use regular 4WD for other low traction surfaces where the differentiation isn't wanted or needed.
Regular 4WD will really compromise handling in turns. The sharper the worse and the longer the wheelbase the worse.
To see what I'm referring to put your truck in 4A and drive around the neighborhood then put it in standard four-wheel drive and drive around the neighborhood and you will feel the front end binding and the handling and steering being compromised.
Snow alone may not be enough to allow front/rear slippage if you're tires can get through a dry snow.