I doubt you can buy any springs to stiffen the rear and I doubt there is anything you can do for the diffs. Try and just stiffen up the rear shocks by putting something in the springs so they can't travel as much. You don't want weight transferring to the rear of the car as much if you want the rear end to slide out. I don't know what you could do, maybe take some rubber bands and wrap around the inside of the springs so they would prevent the springs from going up and down as much but they would still absorb some of the bumps.
I really don't know. that's the only thing I can think of. I would say you will just have to play with it as is. If you can figure out how to stiffen up the rear shocks though it should help a lot but it's going to make it really hard to control a slide. Without oil diffs or a center diff there is really no way to control how much power is going to the front wheels. If your wanting to have a car that you can tune and actually drift you will need to buy a higher end rc car. Probably something at least in the 300$ range. Go to RCtech and take a look around their forum there is everything you would ever want to know about RC cars there.