I have no weight in the middle of my car and about 70 grams underneath my servo and I haven't noticed my x2c nose diving
I would say that it might be your shocks are too soft at the back and the rear is slapping the jumps on takeoff causing it to flick the back end up.
I believe battleaxe on this forum does the weight kit for the x2c but he did mumble about doing one for the monkey chassis to as the x2c one obviously won't fit.