You might want to try a little heavier oil in the rear diff. Maybe go up to 10K. Just like AKSHUN said, there is a lot of trial and error, but this is how we learn. Softer rear springs should add grip to the rear as well. Or harder fronts. Remember that set up is a comprimise, when you improve one area you might loose in another area. Also try different shock mount locations. Try and lay the rear shocks at more of an angle. Different rear tires or a softer compound will also help. I never ran rally before. You have a bigger balance to find since you are probably running dirt and asphault, smooth and bumpy.
I suggest to try one thing at a time, and even go to extremes to feel the difference easier, then back it off a bit. It takes a bit of time, but you will learn you car very well. Also, try different amounts of brake to the front and rear if you have twin brakes.
You mentioned that you adjust camber for full contact, does this mean full contact on the straights or the corners? I would add a little bit of negatiive camber to the rear for more grip in the corners. Good luck!