Downshifting a transmission vs. engaging 4WD on slippery surfaces is two entirely different things with two entirely different outcomes. A sudden downshift of the transmission (especially at a speed where the gear downshifted into shouldn't have been selected) not only causes a rapid gain of RPM's, but also a sudden weight transfer which can and will cause a rear wheel drive car to fishtail. Engaging the transfer case from 2 Hi to 4 Hi at any speed doesn't cause any of this and therefore will not cause a vehicle to fishtail or break traction.Finally some common sense! Shifting at unsafe speeds (determined by the road condition) is never advisable. I've down shifted in my old muscle mopars on rainy roads and have fish tailed all over the place. Slippery conditions warrant more brain cells working and slowing down before drive train configuration changes.