I've no experience with your rig, so can't answer directly. But back in my youth I raced sports cars, and I know that tire pressure makes all the difference in the world with how a vehicle handles, with under inflated tires causing just the problem you discribe. I also know this from driving a truck full of firewood that I borrowed from a friend. On the way to get the firewood, it handled fine. On the way back with the wood aboard, we were all over the road until I stopped at a gas station to pump up the tires. Then we were fine again.
