the limited slip diff if for when you are drinving in a corner the outside wheel is traveling more distance than the inside wheel.

the diff allows the outside wheel to keep up without sliding.

if both wheels are spinning in the same direction then the diff is locked. try lossening the diff a bit till it allows the wheels to spin in opposite directions.

also you might need to lube the diff with diff grease, regular grease will not work.

good luck