You should NEVER drive a vehicle in part time 4wd at highway speeds, whether it's dry pavement or slippery conditions. The transfer case with no center differential is locked on these vehicles which makes them totally unforgiving going around any curve as the front and rear drive wheel on each axle is trying to spin at the same rate. On dry pavement it's hard on the drive train and on snow you can easily loose what traction you have. If the rear wheel looses, you go into a spin.
The only saving grace is that you normally slow down in snow....but, this is a big one, if you have to make a sudden evasive maneuver at highway speeds, you could be in bigger trouble then you ever anticipated. Generally, I use 35 to 40 mph as my upper 4 hi limit. Some newer truck based vehicles with AWD selector switch allows faster snow and dry pavement speeds.
This is why part time systems can be dangerous. If you drive in 2wd with poor weight balance and hit a patch of snow, you can spin as well ! Part time systems are low speed ONLY ! My observation doing accident investigations is that most of the 4wd vehicles I saw off the road in snow were vehicles with part time 4wd that are driven too fast and loose traction while accelerating or turning and start spinning out of control. Inadequate tires exacerbate the situation and make these part time systems MORE of a problem on highways in snow then the quaint little fwd compact.
If you must drive at highway speeds, do it in 2wd ONLY with part time systems exactly like "Uncle Turbo" suggests. ! Newer vehicles with stability control "may" be more forgiving...don't count on it. Some may actually shut this feature off when in part time 4wd. Mine does.