If your sure it's an open carrrier, the additive will do nothing really. If indeed you have an open carrier about the only answer then is... FOD (Foreign Object Debris) floating around inside getting between the moving parts locking things up. This happened to a buddy of mine in his '55 Belair back in 76, wasn't pretty at all. He was doing between 20 & 30 when it completely locked up. Pretty scary ride for about 30 seconds untill all motion ceased.
Wish you the best & be very careful driving, I would get it fixed asap.
Terry
