I seem to remember a previous thread that you had started about this problem when you had asked opinions about the True Trac, it'* possible that your original problem was not with the OE locker but with the ring and pinion gears, I'm far from a diff expert but if it was me I would take it to a shop that specializes in differentials where they'll check the ring and pinion gears for wear, proper mesh, lash, etc. I'm leaning towards the ring and pinion as you had stated earlier that while under torque there was no noise, just while backing off. Sounds like gear slop to me.
2004 Bonne SLE
1971 Chevelle SS 12 bolt with 210k original miles