I am guessing there is more than just shorter shocks and slick tyres to converting the HPI rally car to a road car. Yes too much neg camber can cause loss of traction, but only really if there is not much chassis roll on the car, ie if there is not much traction and the suspension is stiff and does not have much travel, and perhaps if you are running a low speed motor, 0 degrees camber is usually best. I don't think this is it.

If the car is suddenly hooking around as you back off for corners, but is okay when you keep the throttle on, then your inside rear might be lifting off the ground..... ie there is too much weight transfer to to front of the car when you back off. In this case, I would try really stiffening up the front of the car with heavier springs and oil, and check to see if you have got enough droop (down travel) on the back shocks to keep the tyres in contact with the ground. Also, make sure the front diff is not too loose

If the problem however, is as you are getting on the throttle, I would say soften up the rear shocks, although, as it is a rally car, I can't really see this being the case.