My cuda (slap stick) has an 8 3/4 sure grip rear end. I did a little power breaking the other day and noticed only the right rear tire would spin. In the past, both tires spun as they should. If I put the car up in the air, start it, put it in drive (or reverse), both tires spin in the same direction. If I have the car in neutral and I spin the tires by hand, they both spin as they should. I have no idea how old the fluid is in the rear end, but it is not low. Thoughts??

sounds like the clutches/cone might be in the early stages of being worn out.

I had the same problem...
What I did is,
If it's a Clutch Type.
Change your Diff. oil and do not add Sure Grip Additive yet,
Go for a road test,
If both tires spin add 1 ounce of additive at the time to eliminate chatter (too much additive cause slip).
This recipe work for me
If you have no change your clutch are worn out.

If it's Cone Type.
You can try it but I never did it myself

Just to show my stupidity............ how do I know if it's a clutch or cone type? All I know is it's an 8 3/4, 355 gear sure grip.