An open differential drives both wheels - BUT - there is a caveat that many people don't know about.
Read this post by Jerry Bransford:
Why do I need a locker? I thought I had 4WD!
In short, open differentials develop torque dependent on traction - both tires must have traction (be able to grab the ground) for torque to develop. This torque is always split 50/50 between both sides of the axle with an open differential. Development of torque depends on resistance to the tires spinning. Now comes the caveat
- the amount of torque developed (which is split 50/50 between the two tires) depends solely on the amount resistance put on the fastest spinning tire. If one tire on an axle spins fast with little or no resistance (poor traction), it's axle partner (the tire that isn't spinning and has good traction) will get little to no torque. No torquey = no movey.
The mechanics of the open differential are great for the street, but proves to be a limitation, and the Achilles Heel, of 4WD vehicles. It's why enthusiasts opt for lockers of some sort.
And just to clarify - Posi-lok does not lock the differential. It decouples the passenger side axle from the differential manually, instead of using the vacuum operated CAD system.
If you are interested in the mechanics of an open differential you might want to watch this video: