Posi and limited slip are the same exact thing. Positraction or positive traction was a name GM gave their limited slip differential.
If you want the tires to always spin at the same speed, you will need a spool. I would not recommend a spool for anything but a track car as it will seriously screw up the handling of the car.
99 Chevrolet Camaro Z28 M6 - 6.676 @ 103 in 1/8, 10.512 @ 130.2 on street tires, H/C/I
93 Eagle Talon TSI AWD 5spd - Built 6 Bolt, 16G Evo3, HKS 272 Cams - under construction
94 Mustang GT - 5.0 bored and stroked to 347, Vortech V1 @ 10 PSI, Tremec TKO 600 - PIA