Part of the problem with the poor handling of your bike when loaded on the rear may be that your bike is set-up so that the riders center-of-gravity is biased towards the rear already with the saddle pushed back on a set-back seat post and a short stem. You might try pushing the saddle all the way forward and/or use a zero set-back seat post in combination with a longer stem. This will move your body weight towards the front wheel to off-set the rear load.