That's a great illustration. But see how the handle is below grade in the down position?Here is something to consider.
..and difficult to build with stiff enough pivot points so axles stay horizontal..
Not really how you'd want a chicken tractor,I'm also thinking of those lumber carts at Home Depot / Lowes. The high wheel is in the middle ... and there are smaller wheels on the front/back so that it can tip either way.
but putting the wheel in the middle rather than at the end is a good idea to reduce load.