torque is always equal side to side in a differential. In an open diff if one tire is one ice and only producing 10ft/lbs of torque the tire with traction will only get 10ft/lbs of torque which=no movement. A limited slip will not slip until about 150ft/lbs of torque so it will get you moving by sending the 150ft/lbs to the wheel with traction. Hope that makes sence...

 

Matt,

If you want to check that the vacuum system is working, turn the truck off and reach under and grab the cv axle. It should be locked when you try to rotate it by hand. Turn the truck on and grab them again and they should spin freely in 2wd. In 4wd they should lock again. If all that works and it still won't work then it is the shift motor on the transfer case. Might have to wack it with a hammer and or check to see if it is engaging.