They were 0º rear hubs.
That's a pretty big split between front and rear springs for most rubber setups, have you tried a stiffer rear spring or a more progressive one? Maybe the rear suspension isn't applying enough pressure to the actual tire causing it to never really "bite" and thus feel "slippery".
You could try lowering the inner rear hingepins. That should cause the rear end to pressure the tire more, giving you more sidebite, while at the same time rolling less. This may take away corner speed if the back end is already locked down but if it's not like what you're feeling now I think it will just make the car better.