I have an 01 Ram 1500 5.9 4x4. I notice most of the time its a one wheel wonder and sometimes both rear wheels spin.

I was wondering what has to happen for both wheels to lock or is it just random?

 

when both wheels have "equal" traction, you have an illusion of them being locked. this is because the power to the side gears and spider gears is balanced. as soon as one wheel loses traction, then the balance is upset, and all power goes to the spinning tire, and none goes to the wheel with traction, and you are stuck.

i'm looking for a demonstration graphic...
http://static.howstuffworks.com/flash/differential.swf