[1969 GMC]

The locker and posi both help you gain traction, but they are not the same. Any kind of locker will "lock" the axleshafts together, so they spin at the same speed. Some lockers allow the axleshafts to become unlocked when turning, so the wheel speeds can differentiate. Some lockers are engaged with the flip of a switch or push of a button, and will behave like spools (permanently connects the 2 axleshafts) until they are unlocked. Sometimes the axles wont always unlock, and lockers are known for accelerating tire wear when driven on the street.

Posis (or limited slips) have some sort of device inside the carrier to make the axleshafts connect to spin the same speed, but they do not work all the time and can sometimes even get worn. The most common posi uses clutch packs to make the axles resist moving at different speeds. There are other types, such as gear driven and governor driven. Posis generally don't have as bad of handling characteristics as lockers.

I know I am probably leaving a few things out, but hopefully this helps.