PosiTrac is a 1:1 car clutched limited slip differential. it does in no way guarantee equal power being sent to both wheels, it just helps distribute it more evenly. locking the diff is a BAD IDEA. your car will handle terribly, and if you tag a curb with a rear tire, your other tire can't spin to make up for the one being impacted not being able to spin, so you'll break drivetrain parts.
get some silicone diff fluid and fill your rear differential with 3,000 wt and front diff with 7,000 wt fluid-you'll be happy you did