I'm also fighting to find rear end traction on low traction carpet. I have bushing "1 down/in" (maybe it's too much?) and could get it quite good when I find best droop/ride height combination. But still strangely can varies from day to day, tire condition, tire additive etc.
I don't remember exactly what I had in rear diff, but I changed oil to 2000 CST. By feel it felt like it made it softer. And I didn't like it at all. I'm planing to go back to 5k or 7k or even 10k. I see most setup sheets have 5k or more for low traction tracks.
What do you think, is it typical to go higher on diff oil to get rear end more stable?