2 degrees of negative camber on the rear is a bit much unless your hitting berms in every corner. 1 degree to .5 degrees will give you a better contact patch especially in a straight line. Also check your roll center... when you accelerate the rear suspension compresses you could have to much negative camber roll giving you less tire contact with the ground.

1k in the diffs can actually make a car looser. Sense your running 1k in the rear already I sujest going to 3k. I don't know how the rex-x handles but if it feels like the rear end is just swinging in the wind than its to light. If you feel both rear tires are breaking loose of traction because of to much power than lighten the rear diff oil.... you could make the center diff lighter too and you'll get more power to the front tires.

I have made quite a few posts a while ago with how the diffs work... I guess I'll go find them and post them.