In another post about rear axles, there was several people who swore that their rear axles were limited slip units because they saw both tires spin together in a peel-out. An open differential naturally splits the engine torque 50/50 when the outputs have equal resistance. If all four wheels are on the ground and all four tires have equal resistance to spinning then all four wheels will recieve 25% of the applied engine torque, even if you have an open differential at the transfer case and in both axles. In practice the resistance of the brakes, bearings, wheel inertia, and U joints provide enough consistant resistance to always see a split in torque, even when one tire is in mud and another is on pavement. Thats especially true at speed because there is alot of inertia at each wheel. In practical application there is a limit to how much faster one tire can spin than the other. Usually the only way your all wheel drive won't function is if the car is totally stopped, a jack is inserted under one wheel, and a single tire is able to spin fast enough that the other tires don't need to.
If you are going to spend money on low speed traction in severely slippery conditions, the first step is actually to put a limited slip rear differential in. Then the second upgrade would be a low range transfer case. Third would be a new viscous coupling. Fourth would be a full time electric locker front and rear. Fifth would be a solid front axle. Sixth would be mud tires that vibrate terribly at highway speeds because of the obnoxious tread. Eventually you will have an awesome mud vehicle that your other Eagle can tow to an offroad park to play with. Point is that the viscous coupling is an upgrade equivalent to a limited slip rear differential that new Eagles enjoyed from the factory.