I like to share my experience in this DO NOT PUSH dilemma. It is really simple:
You can damage the whole box since it was not designed to be pushed. It's just like a shoe box: you can stack one the top of each other but you just can't push it.
The whole truck suspension is designed to go forward BUT NOT to be pushed; PULL and only PULL. You can total your truck by pushing directly into the box or rear axle.
I've learned this hard way, and I'll tell you what happened to my truck. A few months ago, I got to my destination and started to raise my box when i noticed that my whole box was leaning to one side to the point where it would flip to the side. To prevent it from flipping I dropped the box down by releasing oil supply, but the box was still full of dirt so the box slammed down, my main oil line exploded and of course I wasn't able to raise the box and I got stuck and stuck for gooood.
So i called a tow truck to winch me out and one of the loaders told me that he can push me out. I told him "no you can't push a truck from the back but if you can pull me from the front that will be great." So he called out a different dozer CAT D6 that had pulling cable and he started to pull but wasn't able to pull me out and that's with me helping with my truck. So the other guy that was offering me help by pushing me from the back decided to push me from the back with his own D6.
So they got me out of there and kept pushing me and pulling at the same time and rear D6 was pushing and driving fester then D6 in the front of me. I was inside the truck and suddenly my gear shifter fell down through the floor. When they finally stopped I got out of the truck and noticed that my transmission was under my rear axles.
So, long story short, when he pushed me from the back he pushed too hard and my rear axle shifted forward and pushed drive line into the transmission and busted my bell housing, and the transmission dropped on the ground, broke the U-joint, and rolled under my truck damaging my two tag axles, ripping off my all air lines and wires, and damaging my fuel tank, air tanks, and frame.
So my truck is destroyed; DO NOT PUSH YOUR TRUCK FROM THE BACK EVER !!!