I towed my Jeep Liberty CRD with a Penske trailer behind a Penske E450 box truck. That trailer put so much tongue weight on the truck that I slid through a red light with the front tires locked up just trying to come to a casual stop. Penske of course required that they setup the trailer. I got home and put the jeep on backwards(to get the diesel motor weight over the trailer tires) and put some wood blocks between the tires and chocks to shift it back. Still was pretty tongue heavy even after that but at least I could stop.
Anyways, just be careful, those trailers scare me now.
If I had to guess, the problem was only partly tongue weight. More an issue that it is compounded by the amount of rear overhang that a box truck has. That is the distance from the rear axle to the hitch/ball on a box truck can be really long such that it gives the trailer more leverage.
This is such a key dimension most are unaware of. More important that wheelbase because that's a simplification for the masses. It's really about wheelbase versus rear overhang. Which is why a 5th wheel or semi truck trailer is more stable. It has no overhang to give leverage to the trailer, as the ball is directly over the rear axle.
In setting up any tow vehicle, you want the ball as close and tight to the bumper / rear axle, as possible. Avoiding any unnecessary extra projection to clear things like tailgates, swingout tire carriers, etc. These really compromise towing performance for things like you described and also sway.