8 3/4 rear

OK will a sure grip be locked all the time or just sometime. Sometime when i do a burn out i can leave 2 beautiful lines on the street and then other times just one most of the time just one. Do i have a sure grip thats just wornout or something. Rear came out of what looked to be a 72 to 74 Dart with a locked up 340 which was removed when i went back to get it.