A negative differncial. Well, I gotta say that's a new one on me. I'm sure someone will know though, good luck.
If your talking about how to make the rear end a posi unit. You would have to buy a complete posi third member. They are hard to find and expensive. Some try a mini spool, but I don't really reccommend it on the street. There's different thoughts on that. Some times they are available on ebay.
if your diff is a 8" then you can get a posi for it. there are several brands out there. the auburn is the most popular and is around $400.00. you can find them in mustang II's in the junkyard if you can get lucky and find one.
Posi-Traction is a GM term. Ford made Traction Loc. Chrysler made Sure-Grip, Detroit Locker and Powertrax are more street friendly
