ok so ya i got a 92 3.9 4x4 .
now does limited slip mean that both tires will turn once one starts to spin ?
and if so are they expensive would it be hard to put a lsd in my truck ?

 

with LSD both tires will spin, it shouldnt be hard to put a LSD in urs, find one from a wrecker form a 1st gen and it should bolt right in,

 

Even Late model rears will bolt in I believe. I measured the spring perched today for mine and they are the same. The late model rears will even give you an extra inch on either side. This is good if you wanted to run a wide tire, giving it an extra inch of clearance before rubbing the springs. The white paper tag on the axle tube will tell you if its Limited Slip or not.