Posi Clunking When I Make Sharp Turns

Don't crucify Me. This Question is about My Falcon Sprint. -- I recently had the rear rebuilt. It's a Ford 8" - 3.80 ratio - Posi. When I put it back in, I used Castrol 75-90 gear oil. This oil, supposedly, has anti friction material in it, plus I put in a small tube of friction modifier. -- I drove the car around town, plus a 25 mile round trip, and everything seemed OK. -- Yesterday, I drove it a few miles, and every time that I made a sharp turn (both ways) it made a loud clunking noise. -- My assumption is, that there's a problem with the clutches, in the posi, and I'll have to pull the rear, again, to deal with it. -- Your thoughts, please.