My 69 has a 10bolt rear with 3.73 posi gear. I've noticed that when I am in a parking lot and I cut the wheel hard, that the right rear wheel makes a groaning/grinding sound. Same thing when I cut the wheel hard and back up. It's been doing this for a while but today I noticed it when I took off in a straight line. What's likely to be the cause?

Pop the rear drums off and shoot everything good with brake parts cleaner. Work the adjuster in and out a few times. Are the shoes worn?

Good luck!

Change the oil in the rear end. The problem you are having is very common with the posi differential. You need to put in a bottle of GM oil additive #1052358 and fresh 80W90, or you can use Amsoil 75W90 Series 2000 without the additive. This will take care of your problem. The oil additive allows the clutches in the posi to slip smoothly and stop making the noise. You also need to keep a close eye on the air pressure in the rear tires. If one tire has more pressure than the other that tire will be larger and turn less times than the smaller one. This causes the clutches in the differential to slip constantly while you are driving and will cause the clutches to wear and also overheat the oil. It is very important that both rear tires are exactly the same size. If one tire has more tread wear than the other it will cause the same problem.

I had the same problem with my posi, Drain the oil out and put fresh gear oil with a posi additive.