The "Limited Slip" would be the Ford Traction-Loc diff. "Posi" is actually a GM designation that gets used by everyone else, but is essentially the same thing. 3.08 is high, but will give you great drivability and mileage. If you are looking for a good performance gearing, I would look between 3.55 and 4.10. This would depend on your needs.
--Mike / Turn Ten Racing, Inc.