8 inch posi unit???

I bought an assembled 8" off ebay two months ago. It's in the car already but too cold to take out and try. With the wheels off the ground, both wheels turn in the same direction; somethng I'm not use to. Bought it (third member) for $600 fully assemble limited slip rear with 4.11's.

It is a Ford Trac Lok unit. Tons of people use them. They replace the open carrier in the rear end and allow power to be sent to both tires. Clutch type posi units will wear out eventually though.
If you are a newbie and plan on installing it yourself, I would suggest doing what Geostang did and get the entire 3rd member with your choice of gears already installed and set up. That way it is as easy as removing your center section and bolting in the new one.