factory lsd
First off welcome to FTE lots of good people and info around here.
IMO i would go for a new aftermarket unit if your budget will allow. The factory LSD is fairly weak and not very aggressive for a LS. As for wether or not to go with and LS or a Locker it depends what you use your truck for and how much you off road or tow.
If you off road some and tow some, then an LS is a better choice than a locker as lockers can cause handling issues when towing. If you off road alot and still want to tow a fair amount then you can go with a selectable locker (ie ARB or Elocker)
For and LS the strongest and most bulletproof is the Detroit Truetrac, it is the torsion syle.
So you have to decide what you use your truck for and that will tell you wether to go with an LS, automatic Locker (Detroit) or selectable Locker.
Hope this helps


