8.75rear question

I have an 8.75 in a 73 Dart. Rear wes narrowed, Backbraced,Etc. New 3.91 gears. New Moser axels. Cone type Suregrip. Rear was assembled by a reputable machine shop. The car is a roller. The problem is the Suregrip seems to be locked up. When we moved the car it rolls straight back and forward. but when turning wheels will not turn seperatly. one or the other just slides. Need HELP. Thanks, Al.