Ford's tru-trac wasn't designed to act like posi traction, rather it was designed to shift the traction load from one wheel to the other when necessary. If The Bullet mustang had a "Detroit Locker" (Fords version of posi), it would have burned rubber off both tires going either forward or in reverse. I'm guessing the Bullet Mustang wasn't equipped with what was a fairly pricey option at the time.
.... The Detroit Locker is a 100% automatic locking differential that provides ultimate traction for mild to extreme vehicle applications. Its unique, bullet-proof design is engineered to keep both wheels in drive mode – even if one wheel is lifted off the ground. It works in forward and reverse, and features unmatched strength, 100 percent automatic locking, and maintenance-free parts.