you need a proper gearbox geezer to fit as it involves getting into the box and dismantling things!!
easiest way is to remove the box yourself & take it & the diff to a specialist to fit
as for what it does, it simply transfers power from the spinning wheel to the wheel with traction
e.g. currently if you go round a corner and floor it the inside wheel will spin and you wont go anywhere fast
with a limited slip diff, it transfers the power from spinning inner wheel to outer wheel so you do go somewhere faster!!!
