Just a wild guess, here ---- but I'm taking a stab that the expression "true 12 bolt" is a reference to the larger/stronger GM axle that actually has 12 bolts securing the ring gear to the carrier inside. There is another rear axle, of the Oldsmobile persuasion, that has 12 bolts on the rear cover but internally only has 10 bolts on the ring gear - so in a manner of speaking it's not "a true 12 bolt" despite having a 12 bolt cover.
Posi, limited slip, safe-t-track, etc etc. all refer to an internal differential that's designed to power both rear wheels "most of the time" instead of just one, but you probably understood that already. "Posi-traction" is Chevy's brand name for limited slip, "Safe-T-Trac" is Pontiac's, "Sure Grip" is Chrysler, I think "Traction Lok" might be Ford's, but they all refer to the same thing - a limited slip rear.
So "posi" and "limited slip" are the same thing. The only thing you could do with a welder would be to weld the spider gears in the differential up solid, turning it into a "spool" --- a straight solid axle -- you wouldn't want that for the street.